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.

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, and the second is subdivided in 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.

We note that these descriptions are for the full programs as designed, and not all of the data for each of the sub-programs may be available in DR16.

Catalogs & References

Several papers describe the general targeting algorithms used for the survey. These papers are described in the Technical References. Generally, the key reference for general targeting are Zasowski et al. (2013) for APOGEE-1, Zasowski et al. (2017) for APOGEE-2, and updates for the final years of the survey will be presented in R. Beaton et al. (in prep.) for APOGEE-2N and F. Santana et al. (in prep.) for APOGEE-2S. In addition, several sub-programs have their own dedicated references, which are indicated in Technical References and also included below with the sub-program description.

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

How a Plate is Made

Each APOGEE plate has 300 fibers split into three types of targets. First, we pick telluric targets to estimate atmospheric absorption lines, then we select our science targets, and then at a third priority, we select our sky targets to estimate unresolved light across the field-of-view. The original list of targets from each type is sorted by priority, which implies that if any pair of fibers are incompatible for plugging the lowest priority fiber is rejected. All potential rejections are checked until we have the desired number of fibers of each type in the plate.

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$''$.

In the case of APOGEE-2S, we place acquisition cameras at the center (on-axis) and outer (off-axis) regions of each plate. 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$'$.

Each field in an APOGEE observing program includes one or multiple sets of stars to be observed individually 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. 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. Cohorts were designed to make optimal use of our time by avoiding observations of bright stars for more time than needed to obtain the goal S/N value. Using cohorts, we can obtain relatively similar total S/N values over a wide range of magnitudes. For this reason, the brightest stars are grouped into short cohorts, medium brightness stars into medium cohorts and the faintest stars are grouped into long cohorts.

APOGEE Target Bitmasks

To track the reason why each target was selected for observation we use target flags, which are also called bitmasks. If you are not familiar with bitmasks, please see our bitmask primer.

The target bitmasks used in APOGEE-2 are called APOGEE2_TARGET1, APOGEE2_TARGET2, and APOGEE2_TARGET3. The target bitmasks used for APOGEE-1 targets are APOGEE_TARGET1 and APOGEE_TARGET2 (and an unused one called APOGEE_TARGET3). The right-hand navigation bar has links to bitmask descriptions. 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 allows users to quickly select Main Red Star Sample targets, which have no EXTRATARG bit set (i.e., EXTRATARG==0).

More information on how to use Target Bitmasks is on Using Targets/Samples 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 CO$_{2}$, H$_{2}$O, and CH$_{4}$ absorption bands and OH airglow emission lines. The APOGEE 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, and, 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 take into account spatial variations in the telluric absorption. The procedure is as follows: (i) the FOV of each field 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.

These stars have bit 9 set in APOGEE_TARGET2 or APOGEE2_TARGET2.

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. See the "Observation Files" section of the 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.

Order of Operations

Generally, fibers are assigned in the following order:

Target Class
Description
Telluric
See description here.
Special
Targets provided for specific goals, e.g. star clusters or ancillary programs. These are selected based on their input priorirites and then evaluated for fiber collisions.
Main Red Star Sample
Following the rules described below.
MaStar
APOGEE-2N only. Observations made with MaNGA fiber bundles of calibration stars as well as spectro-photometric standards and sky observations. See MaStar Observing Strategy.
Sky
See description here.

During the APOGEE-2N Bright Time Extension programs, 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 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.

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 science 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 bitmasks.

APOGEE Main Red Star Sample

The APOGEE Main Red Star Sample was designed to select giant-type stars using color-magnitude selection criteria. The sample was designed to be able to map from the observed distributions to the parent sample at confidence, which necessitates a clearly communicated strategy. 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 described 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 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 flags, 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, 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 APOGEE-2's science fibers allotted to each type of cohort.

Dereddened Color Selection

For color selection, we use dereddened colors. We apply a reddening correction to each source based on its $E(H-4.5\mu m)$ color excess (using the Rayleigh-Jeans Color Excess method, RJCE; Majewski et al. 2011, if $4.5\mu m$ photometry is available from either Spitzer or WISE) or on its $E(B-V)$ reddening value in 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 and stars on certain commissioning plates. Comparison to stellar atmospheric and Galactic stellar population models indicate that within APOGEE's typical magnitude range, a color limit of $(J-K_{s})_{0} \geq 0.5$ mag substantially reduces the dwarf contamination in the final sample.

Main Red Star Sample Science Programs

The specific strategy used in a given field depends on the structural component of the Galaxy being targeted: 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-1 bulge stars were selected using a single color limit of $(J-K_{s})_0$ $\geq$ 0.5 mag. For APOGEE-1 targets, bulge fields were selected in the galactic region 357$^{\circ}$ $\leq$ l $\leq$ 22$^{\circ}$, $|b| \leq$ 16$^{\circ}$, while for APOGEE-2 the bulge region corresponds to 340$^{\circ}$ $\le l \le$ 20$^{\circ}$, $|b| \leq$ 25$^{\circ}$.

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 a signal-to-noise 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”.

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. The intended fraction of targets in each color bin is recorded in the apogee2Design file for each plate design. For APOGEE-1 targets, disk fields were selected in the Galactic region 24$^{\circ}$ $\leq$ l $\leq$ 240°, |b| $\leq$ 16°, while for APOGEE-2 the disk region corresponds to 20$^{\circ}$ $\leq$ l $\leq$ 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. This reduced magnitude limit was applied for all the fields were we expected to fill both color bins with the reduced magnitude range; stars selected this way are flagged with APOGEE2_TARGET2 bit 23.

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 proper motion criteria to remove foreground stars (APOGEE2_TARGET2 bit 8). These fields have PROGRAMNAME “odisk” and have “_btx” appended to the FIELD. 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.

Halo

For APOGEE-1 targets, 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). This color range was maintained for APOGEE=2 halo fields. 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.

Often for the APOGEE-2 halo program, we use Washington M, Washington T$_{2}$, 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. 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.”

In APOGEE-2S besides the stars selected using the standard criteria, we explicitly added high priority targets based on spectroscopic and proper motion information, to increase our halo member fraction. Four of these fields are included in DR16 and correspond to 313+29, 294+40, 256+26, and 255-27.

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” is appended to FIELD.

Overlap with MaNGA

APOGEE-2 co-observes with MaNGA using plates that were drilled for 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, instead of the usual $12.2$ mag for a 3-visit plate. Stars observed in a MaNGA-led design have APOGEE2_TARGET1 bit 15 set. All these designs have the PROGRAMNAME tag value “manga”. These fields are predominantly around the "North Galactic Cap."

Filler Targets

If there are any unused fibers in a plate design for any scientific sub-programs -- including Special Programs, then these 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 (e.g., bulge, halo, disk). These targets are selected after targets are asigned fibers for the primrimary scientific goal.

Star Clusters

Targeting in stellar clusters comes in two categories:

  • Confirmed Cluster Members: For well-characterized clusters, members are selected by existing abundance, proper motion, and/or radial velocity measurements. Stars observed for this reason have bit 10 set in APOGEE_TARGET2 or APOGEE2_TARGET2.
  • Cluster Candidates: Cluster candidates are identified solely by their spatial proximity to the central cluster coordinates or by their position relative to the cluster locus in a color-magnitude diagram. Stars can be 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.

Prior to DR16, a set of clusters that had a large number of confirmed members ($\gt$ 12) were used for for internal calibration of pipeline measurements (see < a href="https://ui.adsabs.harvard.edu/abs/2018AJ....156..125H/abstract">Holtzman et al. 2018 for details). These clusters are:

Target Clusters:
Component
Cluster Names
Calibration Clusters
M92, M15, M53, NGC5466, NGC1417, 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 ( apogee2_target2 =2 and 10)
  2. Candidates selected with radial velocities (apogee2_target2=10)
  3. Candidates selected with proper motions (apogee2_target2=10)
  4. Photometric candidates
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, 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”.

Globular cluster candidates may not be flagged appropriately in DR16.

Open Clusters

Open clusters from 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) provides an update for the Open Cluster Chemical Abundance and Mapping survey (OCCAM), including revisions to the target selection using early releses 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
NGC188, NGC2243
APOGEE-2S
NGC2243, M67, NGC2204, NGC2243, NGC6253, NGC5999, NGC6583, NGC6603, Trumpler20, Collinder 261

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. By the end of SDSS-IV, APOGEE-2 will have observed 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 variability.

Cottle et al. 2018 describes the target selection for the APOGEE-2 programs. Cottaar et al. 2014 describes the IN-SYNC program from APOGEE-1.

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."

Targeted Young Clusters:
Survey Component
Cluster Names
APOGEE-1
See IN-SYNC Ancillary Program
APOGEE-2N
Orion A, Orion B, Orion B1, $\lambda$ Ori, Pleiades, Taurus L1495, Taurus L1521, Taurus L1527, Taurus L1536, Taurus L1551 , Taurus L1517, $\alpha$ Per, NGC2264, Cygnus-X, W34
APOGEE-2S
See External Programs

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, for whom less is known about companion systems than for dwarf-type stars. APOGEE-2's substellar companion search focuses on stars with a large number of RV measurements already taken with APOGEE-1 to maximize the number of epochs available at the end of the survey that can be used to characterize companion orbits. More specifically, this program selected fields based on the number of epochs, position in the sky, and diversity of the Galactic environment. These fields are planned to be observed numerous times to reach a final count of $\geq$ 24 epochs for all targets in this class. 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.

While data for this program is included in DR16, the observations are not complete.

Stars targeted as part of this class have targeting bit 4 set in APOGEE2_TARGET3. All designs from fields dedicated for substellar analysis have PROGRAMNAME tag value “substellar”.

RR Lyrae

A number of RR Lyrae (RRL) observations were made by APOGEE-2N. These stars were selected as bright sources accessible with the 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 RRLs have also been observed in suitable fields by both APOGEE-2N and APOGEE-2S. All are indicated by APOGEE2_TARGET1 bit 24.

While data for this program is included in DR16, the observations are not complete.

APOGEE-2S included 10 RR Lyrae fields towards the Galactic bulge in the main survey plan, which corresponded to a total of 25 1-visit designs. However, these visits have been absorbed into the OCIS external program with PROGRAMNAME “kollmeier_17a”.

Spectroscopic Observations of POI’s

Photometric Objects of Interest (POIs) are those stars that show photometric variability. The following programs include stars that have photometry from space telescopes. Besides the 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 Teff≤5500K and log⁡g≤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 (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.

Stars from APOGEE-1 programs are documented here.

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 select K2 Mission Campaign fields, largely from the K2 Galactic Archaeology Program’s (GAP) sample of asteroseismic targets. The details of the GAP sample are given in J. Zinn et al. (in prep.). Several considerations were made in the final target selection:

  1. stars known to host planets
  2. confirmed oscillators in the K2 GAP sample
  3. red giants targeted by GAP, but not observed by GALAH
  4. red giants targeted by GAP, but observed by GALAH
  5. unbiased M dwarf sample

Any remaining fibers followed the criteria for the main red star sample. The stars from this science program have targeting bit 6 set in APOGEE2_TARGET3. All APOGEE-2 designs belonging to K2 fields have PROGRAMNAME tag value “k2” or "k2_btx".

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≤14 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 cadencing 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”.

TESS N-CVZ

As part of the Bright Time Extension, a program was initiatied to study the Northern Continuous Viewing Zone for the TESS satellite. The fields are called "CVZ_*_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 Magellanic Clouds

    The APOGEE-2S Magellanic Cloud (MC) program targets the 12 Small Magellanic Cloud (SMC) fields and 17 Large Magellanic Cloud (LMC) fields. All 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~12.5 mag to H=14.9 mag, and the selection of targets in each field corresponds to a specific combination of several sub-programs targeting different stellar populations in the clouds. A full description of the targeting for the Magellanic Clouds program can be found in Nidever et al. (submitted).

    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 Palomar5, 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, GD1, and Sagittarius 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.

    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”.

    Dwarf Spheroidal Satellites

    APOGEE-2 has targeted a number of dwarf Spheroidal galaxies (dSph, hereafter); more specifically, APOGEE-2N targeted Draco, Ursa Minor, Boötes I and APOGEE-2S targeted Sculptor, Carina, Sextans, and Fornax. The observations for these programs are not complete in DR16.

    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 allocated additional visits. In APOGEE-2S the dSph fields are observed using 24 visits with a single design. Because APOGEE-2S field-of-view is considerably smaller than its northern counterpart, the cohort scheme was not necessary.

    The dSph members 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 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.

    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). In APOGEE-1, five fields were designed for the core of Sagittarius: M54SGRC1, SGRC3, SGRCMA-04, SGRCMI+02, and SGRCNW+02. Candidates for these fields were chosen using the same method and supplemented with kinematic members based on medium resolution spectroscopy from Frinchaboy et al. (2012).

    The APOGEE-2N field, SGRT-1, was targeted similar to that described for the other streams. In APOGEE-2S, four fields were designed for the core of Sagittarius: SGRC-1, M54SGRC-2, SGRC-3, and SGRC-4, and one field to target the tidal tails, SGRT-2.

    The 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. The highest priority stars in these fields are Sagittarius members based on spectroscopic information. 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 supplemented with 2MASS stars with $(J-K_{s})_{0} \geq 0.5$ mag and using the same magnitude limit for separating short and medium cohort stars. Here, the filler sample was restricted to $H \leq 12.8$ mag.

    The Sagittarius tidal 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.

    All APOGEE Sagittarius stars targeted as members based on Frinchaboy et al. (2012) have targeting bit 26 set in APOGEE_TARGET1 or APOGEE2_TARGET1 and the stars that were selected using Wash+D photometry have the corresponding bit according to their classification. All APOGEE-2S designs belonging to the Sagittarius core fields have PROGRAMNAME tag value “sgr”, while designs from the Sagittarius tidal field have the PROGRAMNAME set to “sgr_tidal”.