APOGEE Targeting Information

Every APOGEE field has many more objects in it than APOGEE can observe. Targeting involves the process of selecting which objects will be observed. An object may have been targeted for spectroscopy for any one of many different purposes, or several purposes at once.

Page Scope

The discussion below takes a holistic look at the how and why of APOGEE targets stars. After a general overview, the discussion is divided into sections for each programmatic science goal. The APOGEE project consists of two generations APOGEE-1 and APOGEE-2. APOGEE-2 is divided into APOGEE-2N and APOGEE-2S, which are being carried out from the Northern and Southern hemispheres, respectively. Throughout this section, we highlight the differences between the target selection as implemented in APOGEE-1, APOGEE-2N, and APOGEE-2S.

Catalogs & References

Several papers describe the general targeting algorithms used for the survey. These papers are described in the Technical References. The reference for the targeting procedure used in APOGEE-1 is Zasowski et al. (2013); then, the original targeting method and observing plan for APOGEE-2 is Zasowski et al. (2017); and R. Beaton et al. (2021) and F. Santana et al. (2021) present the final targeting method and observing plan used in APOGEE-2N and APOGEE-2S, respectively, including the modifications performed in the last year of operation of the survey. Several sub-programs have dedicated references that are indicated in Technical References and included with the sub-program description below.

Both astrometry and photometry are required for targeting. The origin of these measurements is recorded in the intermediate data affiliated with targeting, and some of these parameters are maintained in the summary catalogs. In particular, we note the source (H_SRC and PM_SRC) of any targeting information using a code to relate the name of the input catalog. A listing of the catalog codes and links to their related publications is given on Targeting Catalog References.

How a Plate is Designed

Each of the APOGEE spectrographs has 300 fibers that are held to their sky positions using plug plates. The 300 fibers are divided into three types of targets. First, we pick telluric targets to estimate atmospheric absorption lines, second we select our science targets, and third we choose sky targets to estimate sky emission across the field-of-view. The consolidated list of targets from each type is then sorted by priority such that if any pair of fibers are incompatible for plugging, the lower priority fiber is rejected. All potential rejections are checked until we have the desired number of fibers of each type.

Plates observed with the APOGEE-N spectrograph have a 7 square degree field of view (1.5$^{\circ}$ radius), while those observed with APOGEE-S spectrograph have a 2.8 square-degree field of view (0.95$^{\circ}$ radius). After science and calibration targets are selected, we reject fibers based on fiber collisions. A "collision" occurs when two fibers, if placed on the plate, would be separated by less than the size of the protective ferrule around each fiber. When collisions occur, the lower priority target is rejected. For the APOGEE-N, the collision radius corresponds to 71.5$''$ while for APOGEE-S, it is 56$''$.

For APOGEE-2S, acquisition cameras at the center (on-axis) and outer (off-axis) regions of each plate are required. These cameras are used to calibrate the central location of pointing, alignment, and scale, but imply that the areas covered by those cameras are not available for fiber allocation. Thus, candidates in those positions are also rejected. The on-axis camera covers the central 5.5$'$ of the plate, and the dimensions of the off-axis camera are approximately 10$'$ by 7$'$. For APOGEE-2N, there is a central post that supports the plate that obscures the central 96$''$ of the field.

Spectrograph-Dependent Properties of Plate Design

Spectrograph Field-of-View Fiber Collision Radius Fiber Diameter Field Occultations
APOGEE-2N
7 deg2
(1.5 $^{\circ}$ radius)
71.5$''$ 2$''$ Central: 96$''$
APOGEE-2S
2.8 deg2
(0.95 $^{\circ}$ radius)
56$''$ 1.3$''$ Central: 5.5$'$
Off Axis:10$'$ $x$ 7$'$

Each field in an APOGEE observing program includes sets of stars to be observed separately called designs. They are identified using a design id, and several designs can have stars in common with each other. Still, even a difference of a single star distinguishes different designs, and each will be given a unique design id (DESIGNID). Then, a plate corresponds to the phyisical piece of metal used to make each observation. Each plate is used to observe all the stars from a single design and the position of the fibers in them depend not only on the coordinates of the stars, but also the hour angle (HA) at which the plate is going to be observed (because that parameter determines the atmospheric refraction correction terms). For that reason, a plate is equivalent to a design $+$ HA combination, and therefore, a plate contains only 1 design, but each design can be observed using multiple plates (at different HA values).

A design can be further broken into targets in 1, 2, or 3 distinct cohorts, which are a subset of stars restricted to a specific magnitude range. For each field, we make multiple combinations of cohorts to create the different designs. This means that all the stars in a given cohort belong to the same designs, and thus, are always observed together. Cohorts were designed to make optimal use of our time to avoid observing bright stars for more time than needed to obtain the goal $S/N$ value. Using cohorts, we can get relatively similar total $S/N$ values over a wide range of magnitudes. Thus, the brightest stars are grouped into short cohorts, medium brightness stars into medium cohorts, and the faintest stars are grouped into long cohorts.


Schematic representation of how APOGEE uses cohorts. <i>Figure from <a href="http://adsabs.harvard.edu/abs/2013AJ....146...81Z">Zasowski et al. 2013</a>, their figure 1.</i>
Schematic representation of how APOGEE uses cohorts. Figure from Zasowski et al. 2013, their figure 1.

APOGEE Target Bitmasks

To track why each target was selected for observation, we use target flags that are recored using targeting bitmasks. If you are not familiar with bitmasks, please see the SDSS bitmask primer.

The target bitmasks used in APOGEE-2 are called APOGEE2_TARGET1, APOGEE2_TARGET2, and APOGEE2_TARGET3 (and an unused one called APOGEE2_TARGET4). The target bitmasks used for APOGEE-1 are APOGEE_TARGET1 and APOGEE_TARGET2 (and an unused one called APOGEE_TARGET3). The bits within the flags are not exactly aligned between the two generations of APOGEE.

We also include a streamlined bitmask called EXTRATARG in the summary data files. This bitmask allows users to quickly select Main Red Star Sample targets, which have no EXTRATARG bits set (i.e., EXTRATARG==0). More discussion on how to use EXTRATARG is given on Using Targets/Samples.

More information on how to use Target Bitmasks is on Using Targets/Samples and a summary of changes to the bitmasks in DR17 is given in Caveats, and some examples of using bitmasks with APOGEE data are provided in our Data Access Examples.

Telluric Correction Targets

In APOGEE-2, each plate contains 15 telluric fibers, whereas APOGEE-1 plates had 35 telluric fibers.

The APOGEE wavelength range contains several contaminant spectral features from Earth's atmosphere, such as CO2, H2O, and CH4 absorption bands and OH airglow emission lines. The APOGEE data reduction pipeline (Nidever et al. 2015) attempts to remove these features using observations of hot stars to characterize the telluric absorption. To make a telluric absorption correction, we would ideally use a perfect (e.g., featureless) blackbody. Given that hot stars are the best approximation for those, we select the bluest stars in the field. Telluric calibrators are chosen across the full field-of-view to account for spatial variations in the telluric absorption . The procedure is as follows: (i) the field-of-view is divided into equal-area zones, (ii) the bluest star within each zone is selected, and (iii) the bluest stars in the field, regardless of location, are selected. The telluric targets are not dereddened. For more details, see Zasowski et al. (2013).

These stars have bit 9 set in APOGEE_TARGET2 or APOGEE2_TARGET2. EXTRATARG bit 2 is a convenience flag that identifies telluric calibrators across APOGEE-1 and APOGEE-2.


Demonstration of how the APOGEE field-of-view is divided into zones for the selection of calibration fibers. Figure from <a href="https://ui.adsabs.harvard.edu/abs/2013AJ....146...81Z">Zasowski et al. (2013), their figure 8.</a>.
Demonstration of how the APOGEE field-of-view is divided into zones for the selection of calibration fibers. Figure from Zasowski et al. (2013), their figure 8..

Sky Targets

All APOGEE plates contain 35 sky fibers. The data reduction pipeline uses observations of the “empty” sky to monitor airglow. To select suitable empty-sky positions, we select positions that do not have a 2MASS detection within 6$''$. Then, the field is split into equal-area segments, and up to 8 candidates are selected for each zone. The final list of sky fibers is selected randomly from the candidates to ensure uniform coverage across the plate.

The resulting sky spectra could be suitable for the study of the physical conditions, chemical composition, and variability of Earth’s atmosphere. However, these spectra are available only at the single exposure stage, because sky subtraction is performed for each fiber location in each exposure (ap2dvisit). The details of how sky observations are used are given in Visit Combination or Reduction. See the "Observation Files" section of the DR17 Data Access page for how to obtain intermediate data products through the SAS.

These "targets" have bit 4 set in APOGEE_TARGET2 or APOGEE2_TARGET2, but these spectra are not present beyond ap2dvisit (data model).

Order of Operations

Generally, fibers are assigned in the following order:

Target Class
Description
Telluric
35 fibers in APOGEE-1, 15 fibers in APOGEE-2. See description.
Special Targets
Targets provided for specific goals (e.g. star clusters, ancillary programs, or contributed programs). The number of targets can vary from a handful to using all available science fibers. The targets are selected from catalogs based on their input priorities and then evaluated for fiber collisions.
Main Red Star Sample
Following the rules described below. If any fibers are unused on a plate, they will be assigned targets from the Main Red Star Sample.
MaStar
APOGEE-2N only. Observations made are made with MaNGA fiber bundles for calibration stars as well as spectro-photometric standards and sky observations. See MaStar Observing Strategy.
Sky
35 Fibers. See description.

During the APOGEE-2N Bright Time Extension, the highest priority MaStar targets were allocated fibers at the highest priority on the plate (e.g., before APOGEE telluric selection). Lower priority targets, spectro-photometric calibrations, and skies for the MaStar program were assigned in the same order as above. Fields designed in this way have "_btx" appended to their FIELD name. The MaNGA fiber bundles are larger than APOGEE fibers and have a larger fiber-collision radius of 93.6$''$. Extra care was taken in these fields to ensure that this change did not adversely impact APOGEE-2 scientific priorities, but it could impact the Main Red Star Sample selection function (see Beaton et al. 2021 for a discussion).

Science Targets

APOGEE-2 plates have 250 science fibers while APOGEE-1 plates have 230 science fibers. Our science programs are diverse and include many sub-components of our Galaxy and are designed to address many scientific goals. We divide the discussion of the sub-programs thematically. In each sub-program, we have components that were observed as a part of APOGEE-1, APOGEE-2N, and APOGEE-2S; these can be identified via the SURVEY, TELESCOPE, and PROGRAMNAME tags. Within programs, we also have specific target selection algorithms that are described with Targeting bitmasks.

APOGEE Main Red Star Sample

The APOGEE Main Red Star Sample was designed to select giant-type stars using color-magnitude selection criteria. One important motivation for this method was having a simple set of rules permitting a robust reconstruction of the true selection function (see Selection Biases). We describe the general sense of the quantities used for targeting before giving the limits employed for the survey. We also provide brief descriptions of how targeting changed between parts of the survey as indicated by the specific color-magnitude criteria.

General Considerations

Magnitude Limits

The magnitude limits are determined by the number of visits expected for each group of stars and the objective to obtain S/N of 100 per pixel. For each field, there is an associated total number of visits. Long cohort (faintest) stars in a field will be observed every time APOGEE visits the field, while medium and short (brightest) cohort stars will only be observed for a subset of the total visits of the field. The type of cohort in which a star is included is recorded in APOGEE_TARGET2 or APOGEE2_TARGET2 targeting bitmasks, where bits 11, 12, and 13 are set for “short”, “medium”, and “long” cohort stars, respectively. The faint magnitude limits of each cohort are chosen such that the faintest stars of each one will have spectra with a final (combined) signal-to-noise of 100 per pixel and, typically three radial velocity epochs, which implies the following values

H Magnitude Limits:

Number of Visits
H Magnitude Range
1
7.01 $\lt$ H $\lt$ 11.0
3
7.01 $\lt$ H $\lt$ 12.2
6
12.2 $\lt$ H $\lt$ 12.8
12
12.8 $\lt$ H $\lt$ 13.3
24
12.8 $\lt$ H $\lt$ 13.8 or 13.3 $\lt$ H $\lt$ 13.8
1In some disk cohorts, the bright limit was reduced to H=10; stars selected this way are flagged with APOGEE2_TARGET2 bit 23.

Stars with $(J-K_{s})_0$ and $H$ within the relevant limits are then randomly sampled within each cohort. Note that the final total magnitude distribution of spectroscopic targets in a field may differ significantly from the underlying magnitude distribution because the former also depends on the number of each type of cohort in the field as well as on the fraction of science fibers allotted to each type of cohort. Further discussion can be found on Selection Biases.

Dereddened Color Selection

For color selection, we use dereddened colors. Thus, we apply a reddening correction to each source as follows:

  • if $4.5\mu m$ photometry is available from either Spitzer or WISE, $E(J-K)$ is estimated using $E(H−4.5 \mu m)$ from the Rayleigh-Jeans Color Excess method (RJCE; Majewski et al. 2011),
  • otherwise, $E(J-K)$ is estimated from the $E(B−V)$ reddening value from the Schlegel et al. (1998) maps.

The method used for each star is given by its APOGEE_TARGET1 or APOGEE2_TARGET1 bitmask:

Bit
Dereddening Technique
3
RJCE dereddening using Spitzer/IRAC photometry
4
RJCE dereddening using WISE photometry
5
$E(B-V)$ dereddening from Schlegel et al. (1998) (SFD)
6
No dereddening

No reddening corrections were applied for telluric absorption calibrators.

Main Red Star Sample Science Programs

The specific strategy used to select the Main Red Star Sample for a given field depends on the structural component of the Galaxy being targeted; these are: the bulge, disk, and halo. The classification of a field is loosely related to the field position in Galactic coordinates, but there are other considerations. Here we describe the magnitude and color limits imposed for specific structural components.

Bulge

All APOGEE bulge stars (both APOGEE-1 and APOGEE-2) were selected using a single color limit of $(J−K_s)_0$ ≥ 0.5 mag. For APOGEE-1 targets, bulge fields were selected in the galactic region 357≤ l ≤ 22, |b| ≤ 16, while for APOGEE-2 the bulge region was expanded and corresponds to 340 ≤ l ≤ 20, |b| ≤ 25.

All APOGEE-1 bulge fields had one visit, with a faint magnitude limit of H=11 mag.

Given that the Galactic bulge reaches much higher altitudes in the southern hemisphere, all APOGEE-2 bulge fields were observed as part of APOGEE-2S. These fields have a faint magnitude limit of either H=12.2 mag or H = 12.8 mag. Fields designed to a H = 12.2 mag depth were scheduled for three total visits following the standard magnitude visits relation. Due to an underestimation of the time that it would require to complete the bulge APOGEE-2S plan, cohorts in H = 12.8 mag depth bulge fields are not always scheduled for the total number of visits required to reach S/N of 100. In any case, a lower limit of signal-to-noise of ~80 is guaranteed for all APOGEE-2S bulge fields given the number of visits assigned to each cohort.

All APOGEE-2 designs from bulge fields have the PROGRAMNAME “bulge”.

Summary of Bulge Targeting

Survey Range in $b$ Range in $\ell$ Color Magnitude Limit
APOGEE-1
$|b|\leq 16^{\circ}$ $357^{\circ}\leq \ell \leq 22^{\circ}$ $(J-K_s)_0\geq 0.5$ $H\leq 11$
APOGEE-2S
$|b|\leq 25^{\circ}$ $340^{\circ}\leq \ell \leq 20^{\circ}$ $(J-K_s)_0\geq 0.5$ Either:
$H\leq 12.2$ or
$H\leq12.8$

Disk

For APOGEE-1 targets, a single color limit of $(J−K_s)_0 \geq 0.5$ mag was applied in disk fields. For APOGEE-2, a dual-color limit was used, with a defined fraction of the targets having $0.5 \leq (J−K_s)_0 \leq 0.8$ mag and the rest with $(J−K_s)_0 \geq 0.8$ mag. For fields from the inner disk we selected half of the stars in each color bin, while fields from the outer disk ($120^{\circ} \le \ell \le 240^{\circ}$) had 25% of stars selected with $0.5 \leq (J−K_s)_0 \leq 0.8$ and 75% with $(J−K_s)_0 \geq 0.8$ mag. The fraction of targets in each color bin is recorded in the apogee2Design Link! file for each plate design.

For APOGEE-1 targets, disk fields were selected in the Galactic region $24^{\circ} \leq \ell \leq 240^{\circ}$, $|b| \leq 16^{\circ}$, while for APOGEE-2 the disk region corresponds to $20^{\circ} \leq \ell \leq 340^{\circ}$, $|b| \leq 25^{\circ}$.

APOGEE disk fields had depths of $H$=12.2, 12.8, and, in some cases, 13.8 mag. In some APOGEE-2 disk cohorts, the bright limit was reduced to $H$=10 mag to increase the number of faint, and hopefully distant, disk targets. Our original plan was to apply this reduced magnitude limit for all the fields where we expected to fill both color bins with the reduced magnitude range, but due to an error in implementation it was not applied uniformly over regions of the sky (See Caveats); stars selected this way are flagged with APOGEE2_TARGET2 bit 23.

APOGEE-2 designs from disk fields that are not part of APOGEE-2N Bright Time Extension have PROGRAMNAME “disk”, “disk1”, or “disk2”. In APOGEE-2, the "disk1" program is meant to mirror the APOGEE-1 disk footprint, "disk2" are new fields, and "disk" are randomly placed fields.

In the APOGEE-2N Bright Time Extension, a focused effort was made to target substructure in the outer disk. This occurred in two parts: (1) we explicitly targeted confirmed substructure members from previous work (APOGEE2_TARGET2 bit 7) and (2) we identified substructure candidates using reduced proper motion criteria (following the $J$-band reduced proper motion dwarf-giant separation formulated by Collier Cameron et al. 2007) to remove foreground dwarf stars (APOGEE2_TARGET2 bit 8). These fields have programname “odisk” and have “_btx” appended to FIELD.

Summary of Disk Targeting

Survey Range in $b$ Range in $\ell$ Color Magnitude Limit
APOGEE-1
$|b|\leq 16^{\circ}$ $24^{\circ}\leq \ell \leq 240^{\circ}$ $(J-K_s)_0 \geq 0.5$ $H \lt 12.2$ (3-vis)
$12.2 \lt H \lt 12.8$ (6-vis)
$12.8 \lt H \lt 13.3$ (12-vis)
APOGEE-2
$|b|\leq 25^{\circ}$ $20^{\circ}\leq \ell \leq 340^{\circ}$ Two Bins:
$0.5 \leq (J-K_s)_0 \leq 0.8$
$0.8 \leq (J-K_s)_0$
Typical Cohorts:
$H \lt 12.2$ (3-vis)
$12.2 \lt H \lt 12.8$ (6-vis)
$12.8 \lt H \lt 13.8$ (12-vis)

Halo

For all halo programs, a single limit of $(J−K_s)_0 \geq 0.3$ mag was used for halo fields; the bluer color limit was enacted to boost counts because halo-fields have far fewer target candidates).

For APOGEE-1 targets, halo fields were selected in the galactic region $|b|\gt 16^{\circ}$, while for APOGEE-2N and APOGEE-2S the halo region corresponds to $|b| \geq 25^{\circ}$ APOGEE halo fields had depths of $H$ = 12.2, 12.8, or 13.8 mag, to increase the number of distant stars and, thus, halo membership fractions.

The APOGEE-2 halo effort had three targeting sub-programs:

  1. an APOGEE-1 like Washington+DDO51 strategy,
  2. the Bright Time Extension halo program, and
  3. the Southern halo/stream coordinated program.
Washington+DDO51 strategy:

For the original APOGEE-2 halo program, we use Washington $M$, Washington $T2$, and $DDO51$ (Wash+D, hereafter) photometry to classify stars as dwarfs or giants prior to their selection as spectroscopic targets, in addition to the reddening and magnitude limits applied for each field (e.g., Majewski et al. 2000). This pre-selection is employed in these particular fields to increase the selection efficiency of giant stars, which have an intrinsically higher dwarf fraction for APOGEE's magnitude range than for fields in the Galactic plane. Even though a large fraction of APOGEE-2N fields were designed using this “original” strategy, the APOGEE-2S halo program was significantly modified, and thus, only 2 fields in APOGEE-2S were designed in this manner (280+45 and 320+45).

Stars targeted as photometrically classified giants have bit 7 set in APOGEE_TARGET1 or APOGEE2_TARGET1 and are prioritized over photometrically classified dwarfs which have bit 8 set in APOGEE_TARGET1 or APOGEE2_TARGET1. All APOGEE-2 designs from halo fields that are not part of APOGEE-2N Bright Time Extension have PROGRAMNAME “halo.”

Bright Time Extension:

In the APOGEE-2N Bright Time Extension, a focused effort was made to target more distant stars. This occurred in two parts:

  1. we explicitly targeted confirmed K-giants from SEGUE (APOGEE2_TARGET2 bit 20), and
  2. we identified halo candidates using proper motion criteria that removed foreground stars (APOGEE2_TARGET2 bit 21).

Designs from these halo fields will have PROGRAMNAME “halo_btx” and “_btx” appended to FIELD.

Southern Halo-Stream Coordinated Program:

The majority of the APOGEE-2S halo fields were designed to target more distant stars. Four of these new fields exclusively target halo members and candidates (313+29, 294+40, 256+26, and 255-27), while the other 34 new fields coordinated halo and stream observations in the same field by placing fields on identified stream members (described below in Halo Substructures), and targeting other foreground/background halo members and candidates. 26 of these fields targeted the Sagittarius tidal stream, six targeted the JHelum stream, and two the Orphan Stream. Metal-poor halo members (APOGEE2_TARGET2 bit 20) in these fields were selected from Skymapper photometric metallicities, and distant halo candidates (APOGEE2_TARGET2 bit 21) in these fields were identified and targeted using Gaia DR2 proper motions.

Summary of Halo Targeting:
Survey Range in $b$ Range in $\ell$ Color Magnitude Limit
APOGEE-1
$|b|\geq 16^{\circ}$ no $\ell$ limits $(J-K_s)_0 \geq 0.3$ $H \lt 12.2$ (3-vis)
$12.2 \lt H \lt 12.8$ (6-vis)
APOGEE-2
$|b| \geq 25^{\circ}$ no $\ell$ limits $(J-K_s)_0 \geq 0.3$ Typical Cohorts:
$H \lt 12.2$ (3-vis)
$12.2 \lt H \lt 12.8$ (6-vis)
$12.8 \lt H \lt 13.3$ (12-vis)
$13.3 \lt H \lt 13.8$ (24-vis)
Co-Observing with MaNGA

APOGEE-2 co-observes with MaNGA (Bundy et al. 2015) using plates that were drilled for targets from both surveys simultaneously.
APOGEE-2 does not control the location of these pointings and, thus, they are "MaNGA-led." Due to the dither pattern used by MaNGA that results in a flux loss to APOGEE-2's single fibers, the faint limit for MaNGA-led fields is $H \leq 11.5$ mag (0.7 mags brighter than a normal 3-visit plate). Stars observed in a MaNGA-led design have APOGEE2_TARGET1 bit 15 set. All these designs have the PROGRAMNAME value “manga”.

Filler Targets
If there are any unused fibers in a plate design for any scientific sub-program -- including Special Programs, then these unused fibers are assigned to the Main Red Star Sample using the appropriate color-magnitude selection. The color-magnitude selection is set by the number of visits for the plate and the Galactic sub-component suitable for the field as given above (e.g., the ($\ell$, $b$) limits for bulge, halo, disk given above). These targets are selected after targets are assigned fibers for the primary scientific goal.

Star Clusters

Targeting in stellar clusters comes in two categories:

Confirmed Cluster Members
For clusters well-characterized in the literature, members are selected using the extant abundance, proper motion, and/or radial velocity measurements. Stars observed for this reason have bit 10 set in APOGEE_TARGET2 or bit 2 in APOGEE2_TARGET2.
Cluster Candidates
Cluster candidates are identified solely by their spatial proximity to the central cluster coordinates and by their position relative to the cluster locus in a color-magnitude diagram. Stars are selected this way either in poorly-studied or unverified clusters as well as when seeking additional members in well-characterized clusters. Stars in this category have bit 9 in APOGEE_TARGET1 or APOGEE2_TARGET1.

Young or embedded stellar clusters have been targeted differently and are flagged with bit 5 of APOGEE2_TARGET3. Additional clusters have been targeted through Special Programs.

Calibration Clusters
A set of clusters with a large number of confirmed members ($\gt 12$ stars) are used for internal evaluation of pipeline measurements (see Holtzman et al. 2018). These clusters are:

Calibration Clusters
M92, M15, M53, NGC5466, NGC4147, M2, M13, M3, M5, M12, M107, M71, NGC2243, Be 29, NGC2158, M35, NGC2420, NGC188, M67, NGC7789, Pleiades, NGC6819, NGC6791

Globular Clusters

Globular cluster stars from APOGEE were selected independently for each system using the following priority scheme:

  1. Known members based on chemical abundances and stellar parameters determined from prior spectroscopic information ( apogee_target2 or apogee2_target2 bit 2 and 10; bit 2 is specifically used if there are high-resolution abundance measurements for a given member)
  2. Candidates selected with radial velocities (apogee_target2 or apogee2_target2 bit 10)
  3. Candidates selected with proper motions (apogee_target2 or apogee2_target2 bit 10)
  4. Photometric candidates(apogee_target2 or apogee2_target2 bit 10)

Table of Globular Clusters Intentionally Targeted by the Survey:

Targeted Globular Clusters:
Survey Component
Cluster Names
APOGEE-1
NGC4147, M53, M3, NGC5466, NGC5634, M5, M107, M13, NGC6229, M92, NGC6715, M15, M2
APOGEE-2N
M12, M15, M71, M5
APOGEE-2S
47 Tucanae, M10, M12, M22, M4, M55, M68, M79, NGC1851, NGC2298, NGC2808, NGC288, NGC3201, NGC362, NGC6388, NGC6397, NGC6441, NGC6752, Omega Centauri

APOGEE-2 designs belonging to globular cluster fields have PROGRAMNAME tag value “cluster_gc”, “cluster_gc1”, “cluster_gc2”, or “cluster_gc3”.

Open Clusters

Open clusters observed in APOGEE were chosen to cover a wide range of age, metallicity, and galactocentric distance. Frinchaboy et al. 2013 describes the Open Cluster Chemical Abundance and Mapping survey (OCCAM) and includes a detailed discussion of the targeting algorithms. Donor et al. (2018) and provides an update for the open cluster chemical abundance and mapping survey (OCCAM), including revisions to target selection using early data releases from Gaia.

The sense of this targeting is similar to that for the Globular clusters, which is:

  1. Known members based on chemical abundances and stellar parameters determined from prior spectroscopic information
  2. Candidates selected with radial velocities
  3. Candidates selected with proper motions
  4. Photometric candidates

However, photometric candidates are selected such that stars have a common redenning value (see discussion in Frinchaboy et al. 2013). All targets selected in the open cluster program will have apogee2_target1 = 9 .

The complete list of Open Clusters targeted in APOGEE is presented below:

Targeted Open Clusters:
Survey Component
Cluster Names
APOGEE-1
Berkeley 29 (field 198+08), Pleiades, NGC188 NGC2158, M35, NGC2243, NGC2420, M67, NGC6791, NGC6819, NGC7789
APOGEE-2N
Berkeley 2, Berkeley 18, Berkeley 20, Berkeley 21, Berkeley 22, Berkeley 81, Hyades (in K2 C4 and C13 fields), M35, M44, NGC188, NGC2158, NGC2243, NGC2420, NGC6791, NGC 752, NGC7789
APOGEE-2S
Berkeley 75, Berkeley 81, Collinder 261, M67,NGC2243, M67, NGC2204, NGC6253, NGC5999, NGC6583, NGC6603, Tombaugh2, Trumpler20

Some of these clusters were specifically targeted as main sequence calibration clusters to assess and calibrate APOGEE’s performance with M dwarfs at the low-mass end of the main sequence. The following clusters include deep observations to probe their low-mass stars:

Targeted Open Clusters:
Survey Component
Cluster Names
Main Sequence Calibration Clusters
Hyades, M35, M44 (also known as Praesepe, Beehive, or NGC2632), M67

For additional information on this scientific program see:

  • Frinchaboy et al. 2013
  • Donor et al. 2018
  • Donor et al. 2020

All APOGEE-2 designs belonging to open cluster fields have PROGRAMNAME tag value “cluster_oc”.

Young Clusters

APOGEE-2 is targeting several deeply embedded young stellar clusters, to characterize the earliest stages of the older populations that dominate the rest of the sample. APOGEE-2 targeted approximately 200-1000 sources in each of ∼10 embedded clusters. This program is an extension of the APOGEE-1 IN-SYNC ancillary program and shares similar targeting procedures. Targets are drawn from pre-existing catalogs of young stellar objects, identified via their optical/IR photometry, IR excess, X-ray activity, Li abundance, H$\alpha$ excess, or photometric variability.

Note that the ASPCAP pipeline does not include models for pre-main-sequence stars, so the automated synthetic spectral fits are not likely to be meaningful for most of these sources.

Sources targeted as part of the young cluster program are flagged with bit 5 of APOGEE2_TARGET3. All designs belonging to young cluster fields have PROGRAMNAME tag value “yso” or "yso_btx." Young cluster observations were also taken in APOGEE-1 and through additional Special Programs in APOGEE-2.

Targeted Young Clusters:
Survey Component
Cluster Names
APOGEE-1
All young clusters in APOGEE-1 were targeted through the IN-SYNC Ancillary Program
APOGEE-2N
Orion A, Orion B, Orion B1, $\lambda$ Ori, Pleiades, $\alpha$ Per, NGC2264, Cygnux-X, Rosette Complex, W3/4/5, Taurus
APOGEE-2S
All young clusters in APOGEE-2S were targeted through External Programs

For additional information on this scientific program see:

Radial Velocity Programs

The following programs were designed around the radial velocity measurements produced by the APOGEE instruments.

Substellar Companions

Several stars are repeatedly observed by APOGEE-2 to characterize substellar companions; this program focuses on red giant stars, because less is known about their companion systems than for dwarf-type stars. The substellar companion program is built on the APOGEE-1 observations to optimize the scientific return from orbital fitting by maximizing the number of epochs and their time-baseline by the end of APOGEE-2 More specifically, this program selected fields from APOGEE-1 based on the number of visits, location in the sky, and to encompass a diverse set of Galactic environments. These fields are planned to be observed numerous times to reach a final count of NVISITS $\geq$ 24 epochs. Within each field, the stars are selected from those targeted by APOGEE-1, prioritized first by the number of APOGEE-1 epochs and then by brightness, with brighter stars receiving higher priority. Stars targeted as part of this program have targeting bit 4 set in APOGEE2_TARGET3.

Summary of Substellar Companions Targeting:
Observing Mode PROGRAMNAME FIELD
APOGEE-2N subtellar 120-08-RV, 150-08-RV, 180-08-RV, N188-RV, COROTA2-RV
Ancillary anc IC348_RV, M67_RV, M3_RV
APOGEE-2N BTX sub_btx N6819-RV_btx, N1333-RV_btx, N5634SGR2-RV_btx; 090+00_btx, 203+04-RV_btx; ORIONA-RV_btx, ORION-RV_btx

RR Lyrae

A number of RR Lyrae observations were made by APOGEE-2N. These stars were selected as bright sources accessible with the NSMU 1-m telescope at APO and observed for a varying number of epochs. All pre-selected RRL stars have the APOGEE2_TARGET1 bit 24 set. Additional RR Lyrae have also been observed in suitable fields by both APOGEE-2N and APOGEE-2S. All are indicated by APOGEE2_TARGET1 bit 24.

Spectroscopic Observations of POI’s

Photometric Objects of Interest (POIs) are those stars that show photometric variability in precise and high-cadence monitoring. The following programs target stars that have such photometry from space telescopes. Besides the survey programs described here, a number of Special Programs also target POIs.

APOKASC

APOGEE-2 has expanded upon APOGEE-1's asteroseismic program (APOKASC) by completing a magnitude-limited sample of Kepler targets with and without solar-like oscillations. This sample comprises giants with $T_{eff} \leq 5500$ K and $\log⁡ g \leq 3.5$, and dwarfs with 5000 $\leq T_{eff} \leq$ 6500 K and log⁡$g$ $\geq$ 3.5 dex; these pre-observation temperature and gravity estimates come from the revised Kepler Input Catalog (KIC; Huber et al. 2014) and the corrected temperature scale of Pinsonneault et al. (2012).

All APOGEE-2 APOKASC targets have APOGEE2_TARGET1 bit 30 set, with giants and dwarfs being further identified with APOGEE2_TARGET1 bits 27 and 28, respectively, if known. All APOGEE-2 designs belonging to APOKASC fields have PROGRAMNAME tag value “kep_apokasc”.

Additional details of the APOKASC program can be found in Pinsoneault et al. (2014) for APOGEE-1 and
Pinsoneault et al. (2018) for APOGEE-2.
Other References:

Eclipsing Binaries

Approximately 100 EBs were targeted in APOGEE-1, predominantly in the Kepler footprint. In APOGEE-2, this sample is more than doubled to include additional Kepler-detected EBs as well as systems identified in the Kilodegree Extremely Little Telescope survey (KELT; Pepper et al. 2007). The Kepler targets are selected from the Kepler EB Catalog’s list of detached EBs (Prsa et al. 2011, Slawson et al. 2011), using a magnitude limit of $H \leq 13$ mag. A total of ten EB targets are selected in each Kepler field, though not all may be available in the current data release. The KELT-based sample is selected from systems lying in already-planned APOGEE-2 field locations that are anticipated to be observed for eight epochs over the course of the survey. KELT itself is restricted to bright stars, so no additional magnitude cuts are required---simply the presence of a well-defined orbital period, with a further preference towards those systems that have a detached morphology, are bright, and/or have shallow secondary eclipses. Up to a maximum of five KELT targets appear in any given field.

All APOGEE-2 targets from the EB program are flagged with bit 1 in APOGEE2_TARGET3.

K2

APOGEE-2 is targeting several thousand giant stars in K2 Campaigns. The targets are largely drawn from the K2 Galactic Archaeology Program’s (GAP) sample of asteroseismic targets (APOGEE2_TARGET2 bit 0). Details of the GAP sample have been presented in Stello et al. (2017) and Zinn et al. (2020). Several considerations were made in the final target selection:

  1. stars known to host planets (APOGEE2_TARGET2 bit 11)
  2. confirmed oscillators in the K2 GAP sample (APOGEE2_TARGET1 bit 30 and APOGEE2_TARGET2 bit 0)
  3. red giants targeted by GAP, but not observed by GALAH (APOGEE2_TARGET2 bit 0)
  4. red giants targeted by GAP and observed by GALAH (APOGEE2_TARGET2 bit 0 and APOGEE2_TARGET2 bit 17)
  5. Targets from the ancillary program producing an unbiased M dwarf sample (APOGEE2_TARGET3 bit 28)

Any remaining fibers followed the criteria for the Main Red Star Sample.
All targets from this K2 program have targeting bit 6 set in APOGEE2_TARGET3, in addition to flags associated with the targeting category above. All APOGEE-2 designs belonging to K2 fields have PROGRAMNAME tag value “k2” or "k2_btx".

The following campaigns have observations in APOGEE-2: C1, C2, C3, C4, C5, C6, C7, C8, C11, C12, C13,C16, and C18.
Besides these main survey observations, K2 stars were also targeted in three APOGEE-2S contributed programs whose PROGRAMNAME tag values are ‘teske_17a”, “TeskeVanSaders_17b”, and “weinberg_17a”. More details about these and other contributed programs are given in the Special programs section.

KOI

The APOGEE-2 Kepler Object of Interest (KOI) program contains ∼1000 KOIs and ∼200 non-planet-hosts distributed across seven APOGEE-2 fields, supplemented by ∼200 KOIs observed in APOGEE-1. For this program, planet hosts and KOIs were drawn from the NExScI archive using a simple magnitude limit of $H \lt 14$ mag to identify all CONFIRMED or CANDIDATE targets in the fields. The non-host control sample was drawn from the Kepler Input Catalog (Brown et al. 2011), using the same $H\leq14$ magnitude limit and selected to provide the same $T_{eff}$-log$⁡g$ joint density distribution as in the host+KOI sample. These control sample stars are used to fill fibers unused by the host+KOI sample.

Each APOGEE-2 KOI field is observed over 18 epochs, with a cadence sufficient to characterize a wide range of orbits. The host+KOI targets can be identified with bit 0 of APOGEE2_TARGET3, and the control sample targets with bit 2 of APOGEE2_TARGET3. All APOGEE-2 designs belonging to KOI fields have PROGRAMNAME tag value “kep_koi” (Modules 04, 06, 07, 10, and 21) or “kep_koi_btx” (Modules 18 and 19).

TESS N-CVZ

As part of the Bright Time Extension, a program was initiated to study the Northern Continuous Viewing Zone for the TESS satellite. The fields are named as "CVZ_$\ell \ell \ell$+$bb$_btx" and have PROGRAMNAME "cvz_btx."

Targets were selected with a multi-tier priority scheme that prioritized rare targets over more common targets. All targets were selected from the TESS Input Catalog (TIC). The targeting is documented in APOGEE2_TARGET2 as follows:

  • bit 27 APOGEE2_CVZ_AS4_OBAF: OBAF stars
  • bit 28 APOGEE2_CVZ_AS4_GI: targets in Guest Investigator programs such as planet hosts, Astroseismic Target List, Subgiants, and Cool-dwarfs
  • bit 29 APOGEE2_CVZ_AS4_CTL: Filler CTL star selected from the TESS Input Catalog
  • bit 30 APOGEE2_CVZ_AS4_GIANT: Filler Giant selected in a reduced proper motion diagram

The targeting algorithm was designed to target ~50% giants and ~50% dwarfs for each plate, where the dwarf-sample contains stars in both the ATL and CTL.

The Magellanic Clouds

The APOGEE-2S Magellanic Cloud (MC) program targets 12 Small Magellanic Cloud (SMC) fields and 36 Large Magellanic Cloud (LMC) fields. One deep, single cohort, 24 visit field (FIELD of "LMC_VdS") in the LMC was targeted to have targets overlapping with the high-resolution optical spectroscopy from van der Swaelmen et al. (2013). All remaining MC fields have a single cohort single design, with 9 and 12 visits for LMC, and SMC, respectively. The faint magnitude limit of MC fields varies significantly across the program from $H \sim 12.5$ mag to $H = 15.3$ mag, and the selection of targets in each field corresponds to a specific combination of several sub-programs targeting the different stellar populations in the clouds below, in order of priority:

  1. Supergiants following Neugent et al. (2012) and Bonanos et al. (2009) (limited to $\leq 20$ per plate),
  2. Hot main-sequence stars (limited to $\leq 20$ per plate),
  3. Olsen et al. (2011) retrograde stars (limited to $\leq 20$ per plate),
  4. Post-AGB stars from Kamath et al. (2014, 2015) (limited to $\leq 10$ per plate),
  5. RGB stars with high resolution public spectroscopy (limited to $\leq 10$ per plate),
  6. AGB Carbon-rich stars following Nikolaev et al. (2000) (limited to $\leq 20$ per plate),
  7. AGB Oxygen-rich stars (limited to $\leq 20$ per plate),
  8. RGB stars ($\geq 130$ per plate).

A full description of the targeting for the Magellanic Clouds program can be found in Nidever et al. (2020) and Santana et al. (2021).
MC members have targeting bit 22 set in APOGEE2_TARGET1, while MC photometric candidates have targeting bit 23 set in APOGEE2_TARGET1. All designs belonging to Magellanic Cloud fields have PROGRAMNAME tag value “magclouds”.

Halo Substructures

Stellar Streams

APOGEE has targeted a variety of stellar streams that either represents the remnants of galactic mergers, tidally disrupted clusters, or have a yet unknown nature.
In APOGEE-2N, we targeted five streams: the Triangulum-Andromeda (TriAnd) structure, the tidal tails of the globular cluster Palomar 5, the Orphan stream, the GD-1 stream, and the Sagittarius tidal tail.

To observe the TriAnd structure, we selected the 5 fields (TRIAND-1 to TRIAND-5) where the standard halo selection without Wash+D photometry selected most TriAnd candidates from Sheffield et al. (2014) and Chou et al. (2011). Additional targeting in the area spanned by TriAnd occurred in the Bright Time Extension program in the outer disk.

For the Palomar 5, Orphan (in APOGEE-2N), and GD1 streams, we used a variety of catalogs to select likely members, using the following priority ranking:

  1. Stars classified as giants using Wash+D photometry, and photometric candidates using the location in $(J-K_{s})_{0}$ versus $H$ CMD.
  2. $(J-K_{s})_{0}$ versus $H$ photometric candidates without Wash+D dwarf/giant classification
  3. Wash+D-classified giants with lower membership probability based on the $(J-K_{s})_{0}$ versus $H$ CMD location.
  4. Stars without Wash+D dwarf/giant classification and with lower membership probability based in the $(J-K_{s})_{0}$ versus $H$ CMD location.

For the Jhelum and Orphan (in APOGEE-2S) streams, stream targets were selected using Gaia DR2 astrometry and photometry, and 2MASS photometry following the descriptions in Santana et al. (2021) and Sheffield et al. (2021; for details on the Jhelum selections). Jhelum was targeted in six 1-visit, single cohort fields (FIELD JHelum1 through JHelum6), and Orphan was targeted in two fields each with 3-visit short cohorts and a 6-visit medium cohort (FIELD 339-44 and Orphan1).
All stream photometric candidates have targeting bit 19 set in APOGEE2_TARGET1, and the corresponding Wash+D flag according to their classification. All designs belonging to stream fields have PROGRAMNAME tag value “halo_stream”, “stream_halo”, or “halo” in the case of Orphan field 339-44.

Dwarf Spheroidal Satellites

APOGEE-2 has targeted resolved stars in the dwarf Spheroidal galaxies (dSph, hereafter); more specifically, APOGEE-2N targeted Draco, Ursa Minor, Boötes I and APOGEE-2S targeted Sculptor, Carina, Sextans, and Fornax. Only Fornax was targeted after a Gaia data release and was able to utilize proper motions.

All APOGEE dSph fields are scheduled for at least 24 total visits. In APOGEE-2N the dSph fields contain four 6-visit designs, and each field includes four short cohorts, two medium cohorts, and a single long cohort. In the Bright Time Extension of APOGEE-2N, each dSph field was given additional visits.

In APOGEE-2S, the dSph fields were also scheduled for 24 visits, but used only a single cohort. Because APOGEE-2S field-of-view is considerably smaller than its northern counterpart, the cohort scheme was not necessary.

The dSph members from literature have targeting bit 20 set in APOGEE2_TARGET1 and dSph photometric candidates have targeting bit 21 set in APOGEE2_TARGET1. All designs belonging to dSph fields have the PROGRAMNAME tag value “halo_dsph”.

The Sagittarius System

Both the core and the tidal tails from the Sagittarius dwarf galaxy are targeted in several APOGEE fields.

Sagittarius Dwarf Spheroidal

In APOGEE-1, five fields were designed for the core of Sagittarius: M54SGRC1, SGRC3, SGRCMA-04, SGRCMI+02, and SGRCNW+02. In APOGEE-2S, six fields were designed for the core of Sagittarius: SGRC-1, M54SGRC-2, SGRC-3, and SGRC-4, Sgr_rvvar1, and Sgr_rvvar2. Candidates for these fields were chosen using the 2MASS M giant selection process described in Majewski et al. (2003) and supplemented with kinematic members based on medium resolution spectroscopy from Frinchaboy et al. (2012).

The APOGEE Sagittarius core fields have 3-visit short cohorts and a single 6-visit medium cohort, except for M54SGRC-2 that has 6-visit short cohorts and a single 12-visit medium cohort and Sgr_rvvar1 and Sgr_rvvar2, both of which only have 3-visit short cohorts. The highest priority stars in these fields are Sagittarius members based on spectroscopic information (and particularly members with previous APOGEE observations for Sgr_rvvar1 and Sgr_rvvar2). Member stars brighter than $H=11.3$ mag were assigned to the short cohorts, and stars with $11.3 \leq H \leq 14.0$ mag were assigned to the medium cohorts. For the fields that were not filled with Sagittarius members, we back-filled the plate with 2MASS stars with $(J−K_s)_0 \geq 0.5$ mag and used the same magnitude limit for separating short and medium cohort stars. Here, the filler sample was restricted to $H \leq 12.8$ mag.

All APOGEE Sagittarius stars targeted as members based on Frinchaboy et al. (2012) have targeting bit 26 set in APOGEE_TARGET1 or APOGEE2_TARGET1. All APOGEE-2S designs belonging to the Sagittarius core fields have PROGRAMNAME tag value “sgr”.

Sagittarius Tidal Stream

In APOGEE-1, four fields were designed for the Sagittarius tidal tails: N5634SGR2, SGR1, SgrO2, and SgrO3. In these fields, Sagittarius stream candidates were chosen using the 2MASS M giant selection process described in Majewski et al. (2003).

The APOGEE-2N field, SGRT-1, was targeted similar to that described for the other streams, and the APOGEE-2S field, SGRT-2 has 3-visit short cohorts and a single 6-visit medium cohort. The highest priority stars in this field are Sagittarius members based on spectroscopic information and secondary priority are the Wash+D classified giants; all of these stars were assigned to the medium cohort with a magnitude range of $9.0 \leq H \leq 15.3$ mag. Remaining fibers on the plate were filled with 2MASS stars using a color limit of $(J−K_s)_0 \geq 0.3$ mag and magnitudes limits of $8.9 \leq H \leq 12.0$ mag for short cohort stars, and $12.0 \leq H \leq 15.3$ mag for medium cohort stars.

The majority of the APOGEE-2 targeting of the Sagittarius tidal stream was designed following the public release of Gaia DR2 and includes 26 fields in APOGEE-2S. Eight of these fields are named according to their Galactic longitude and latitude and the remaining 18 fields have FIELD sgr_tidal# (1-18; with 1, 2, and 3 capitalized as, e.g., Sgr_tidal1). These fields were all designed with 3-visit short cohorts and a single 6-visit medium cohort. The highest priority targets in these fields were Sagittarius stream members with $10 \lt H \lt 13.5$ mag identified in Hayes et al. (2018), followed by halo members and halo candidates described in the Halo section, and remaining fibers were filled with stars from the main red star sample.

Sagittarius stream members from APOGEE-1 have targeting bit 26 set in APOGEE_TARGET1, and from APOGEE-2 have targeting bits 18 and/or 26 set in APOGEE2_TARGET1 and stream candidates have targeting bit 19 set in APOGEE2_TARGET1. Fields named according to their Galactic latitude and longitude that target Sagittarius stream stars have their PROGRAMNAME set to “halo” and all other Sagittarius stream fields have the PROGRAMNAME set to “sgr_tidal”.