Young Stellar Objects

Infrared IRAS image of the Perseus/Taurus/Orion star-forming region
Image Credit: Thomas Preibisch

In MWM, we use the term “YSO” to refer to the phases of stellar evolution between protostars and the main sequence. Because O and B stars evolve so quickly to the main sequence, they are not seen in these phases and are mainly observed under the OB program. Therefore this program focuses on lower mass stars. The highest priority target classes are those based on all-sky photometry.

Target selection for YSOs is not straightforward, because they show a wide range of physical properties as they move from protostars to pre-main-sequence stars, complicated by nebular emission, line-of-sight extinction, and mass dependence. Kounkel et al (2023) provides the comprehensive discussion of YSO targets through target generation v1.0. The summary here applies to plate observations (targeting generation 0.1.0) and early FPS observations. The majority of FPS observations were part of targeting generation 0.50. Exceptions are noted in the detailed descriptions for the cartons below.

APOGEE and BOSS spectra

Both infrared APOGEE and optical BOSS spectra contain useful information for YSOs. However, some of the cartons discussed below do not have optical magnitudes bright enough to obtain reasonable signal-to-noise in a single exposure. Therefore only a subset of methods have both _apogee and _boss versions for cartons. The selection criteria are the same for both instruments, with the exception of the limiting apparent magnitude and band.

Cartons

Disk

These YSOs are identified by large infrared excesses due to protoplanetary disks. WISE colors are particularly helpful in this regard. The major contaminant are distant field stars that are highly extincted so a parallax cut is included.

During plate operations, both APOGEE and BOSS spectra were obtained under the carton name mwm_yso_s1. During FPS observations, the APOGEE and BOSS spectra were obtained under carton names mwm_yso_disk_apogee and mwm_yso_disk_boss, respectively. Targets can also be identified with the alternate carton name mwm_yso_disk.

Selection Criteria

All bolded criteria below are common to all disk cartons. Some criteria only apply to BOSS or APOGEE cartons as indicated.

  • W1-W2 >0.25
  • W2-W3 > 0.5
  • W3-W4 > 1.50
  • parallax > 0.3 mas
  • H < 13 (mwm_yso_s1 and mwm_yso_disk_apogee)
  • GRP < 15.5 (mwm_yso_disk_boss)

Embedded

These stars are too faint in the optical for reliable parallaxes because they are still embedded in their natal envelopes, have edge-on disks, or have more extinction along the line of sight. Without the parallax constraint, more stringent color cuts on both the AllWISE and 2MASS photometry are needed to avoid extensive contamination by reddened field stars.

The carton was named mwm_yso_s2 during plate operations and mwm_yso_embedded_apogee during FPS operations. The alternate carton name is mwm_yso_embedded for both eras.

Selection criteria

  • H < 13
  • G > 18.5 or undetected
  • J-H > 1
  • H-K > 0.5
  • W1-W2 > 0.5
  • W2-W3 > 1
  • W3-W4 > 1.5
  • W3-W4 > 0.8*(W1-W2)+1.1

Nebula

In regions of high nebulosity, the WISE W3 and W4 bands can become saturated with nebular emission. Therefore, where W3 and W4 information is not available, we use shorter wavelength information. We also exclude certain regions of the sky which produce spurious sources following the scanning law of the WISE telescope.

The carton was named mwm_yso_s2-5 during plate operations and mwm_yso_nebula_apogee during FPS operations. The alternate carton name is mwm_yso_nebula for both eras.

Selection Criteria

  • H < 13
  • (W4 not reported and W2-W3 > 4) OR (W3, W4 not reported and J-H > 1.1)
  • b < 5 deg
  • b > -5 deg OR l > 180 deg

Central Molecular Zone

The central molecular zone (CMZ) is the most extreme star-forming environment in the Milky Way and is hidden behind many magnitudes of optical extinction. This area covers a relatively small fraction of the sky compared to the all-sky nature of the previous selections. There is deeper targeted data from the GLIMPSE and MIPSGAL programs that cover the inner Galaxy, which offer a substantial improvement in sensitivity and resolution compared to WISE. Targets are selected at the Galactic center within 1 degree of galactic latitude and 2 degrees in galactic longitude.

The carton was named mwm_yso_cmz during plate operations and mwm_yso_cmz_apogee during FPS operations. Targets can also be identified with alternate target name mwm_yso_cmz.

Selection Criteria

  • H< 13
  • [8.0]-[24] > 2.5 in the catalog of Gutermuth and Heyer (2015) available on the CDS
  • parallax < 0.2 or not measured
  • -1 < b < 1
  • (l < 358) OR (l > 2)

Variable

YSOs are on average more variable than main-sequence stars of the same color, which provides a way of identifying them when their disks have evaporated enough and no longer provide a large infrared excess. This works particularly well for cooler stars, where the higher activity of young stars leads to more spots and hence larger photometric variability. The multi-epoch photometry from Gaia provides the all-sky data needed for this method.

Following Belokurov et al. (2017), we define the photometry variability in a given Gaia filter x as

    \[V_x=\frac{\sqrt{phot\_x\_n\_obs}}{phot\_x\_mean\_flux\_over\_error}\]

where phot_x_n_nobs is the number of observations that contributed to the photometry in a given band and phot_x_mean_flux_over_error is the mean flux in a given band divided by its error. Both values are from Gaia DR2.

During plate operations, both APOGEE and BOSS spectra were obtained under the carton name mwm_yso_s3. During FPS observations, the APOGEE and BOSS spectra were obtained under carton names mwm_yso_variable_apogee and mwm_yso_variable_boss, respectively. Targets can also be identified with the alternate carton name mwm_yso_variable.

Selection Criteria

All cartons, regardless of instrument used, used the following selection criteria.

  • H < 13
  • G < 18.5
  • parallax > 0.3
  • VG > 0.02
  • VBP > 0.02
  • VRP > 0.02
  • VG0.75 < VBP < VG
  • 0.75 VG < VRP < VG0.95
  • GBP – GRP > 1.3
  • MBP > 5 log10VBP + 11 where MBP = GBP-5*(log10(1000/parallax)-1)
  • 2.5 * (GBP-GRP) -1 < MG < 2.5 * (GBP-GRP) + 2.5 where MG = G-5*(log10(1000/parallax)-1)
  • parallax > 0.3

Pre-Main Sequence

This method relies on finding young low-mass stars by their location above the main sequence on the HR diagram as they continue their slow contraction. There have been several dedicated studies that focused on identifying such stars through Gaia DR2 photometry and astrometry, and we draw targets from two such studies, Zari et al (2018) and McBride (2021), for these cartons.

During FPS observations, the APOGEE and BOSS spectra were obtained under carton names mwm_yso_pms_apogee and mwm_yso_pms_boss, respectively under targeting plans 0.5.0 and 0.5.17. Beginning in 0.5.17, explicit bright limits were removed from the selection criteria. These targets can also be identified with the mwm_yso_pms alternate carton name.

Selection Criteria

All bolded criteria below are common to all disk cartons. Some criteria only apply to BOSS or APOGEE cartons as indicated.

  • Candidate in either Zari et al (2018) or McBride (2021) catalogs. These can be queried via CasJobs in the DR19 context as the mos_zari18pms and mos_sagitta tables or downloaded from the SAS.
  • H < 13 (mwm_yso_pms_apogee)
  • H > 7 (mwm_yso_pms_apogee, targeting generation 0.5.0 only)
  • GRP < 15.5 (mwm_yso_pms_boss)
  • GRP > 7 (mwm_yso_pms_boss, targeting generation 0.5.0 only)

Cluster

A stars collapse quickly to the main sequence but once there, their lives are relatively long. Unlike O and early B stars, observing an A star in the field is not a guarantee that it will to be < 100 Myr old. With neither short lives on the main sequence nor obvious pre-main-sequence phases, we require additional information to find truly young A stars. We rely on the fact that stars are born in associations to find these intermediate mass stars. These cartons select all stars from moving groups with an age < 107.5 yr from the analysis of Kounkel et al. (2020) using the Gaia DR2 data.

During plate operations, both APOGEE and BOSS spectra were obtained under the carton name mwm_yso_cluster. During FPS observations, the APOGEE and BOSS spectra were obtained under carton names mwm_yso_cluster_apogee and mwm_yso_cluster_boss, respectively. These targets can also be identified with the mwm_yso_cluster alternate carton name.

Selection Criteria

All bolded criteria below are common to all disk cartons. Some criteria only apply to BOSS or APOGEE cartons as indicated.

  • Candidate in Kounkel et al. (2020) catalog. This catalog can be queried via CasJobs in the DR19 context as the mos_yso_clustering table or downloaded from the SAS.
  • log(age) < 7.5 log(years)
  • H < 13 (mwm_yso_cluster and mwm_yso_cluster_apogee)
  • GRP < 15.5 (mwm_yso_disk_boss)

Cadences

During plate operations, the YSO program was one of the science programs that could drive the field cadence, and plates with the YSO cadence obtained 3 epochs of observations during bright time. However, YSO targets could have been observed any plate except those with the RM cadence. Note that some APOGEE observations were taken for a “mwm_yso_ob” carton that was later abandoned when it became clear it was fully a subset of the mwm_ob_core carton.

All of the * _apogee cartons use the bright_3x1 cadence. Targets in the *_boss cartons use one of the bright multi-epoch cadences, depending on their GRP magnitude.

  • bright_3x1: GRP< 14.76
  • bright_4x1: 14.76 ≤ GRP < 15.075
  • bright_5x1: 15.075 ≤ GRP < 15.29
  • bright_6x1: 15.29 ≤ GRP < 15.5
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