# Star Formation in the Orion and Taurus Molecular Clouds

## Contact

Jill Knapp
Princeton University
gk@astro.princeton.edu

## Summary

A program to collect spectra of candidate young stellar objects (YSOs) in the Orion and Taurus molecular clouds first seen in 2MASS, Spitzer, and WISE imaging

## Finding Targets

An object whose ANCILLARY_TARGET2 value includes one or more of the bitmasks in the following table was targeted for spectroscopy as part of this ancillary target program. See SDSS bitmasks to learn how to use these values to identify objects in this ancillary target program.

Program (bit name) Bit number Target Description Number of Fibers
25_ORI_WISE 40 WISE-selected target near the star 25 Ori 290
25_ORI_WISE_W3 41 WISE-selected target near the star 25 Ori, selected from data in the W3 band 484
KOEKAP_STAR 42 WISE-selected source in the star-forming region Kappa Ori, selected from WISE infrared excess 252
KOE2023_STAR 43 WISE-selected source in the star-forming region NGC 2023, selected from WISE infrared excess 202
KOE2068_STAR 44 WISE-selected source in the star-forming region NGC 2068, selected from WISE infrared excess 276
KOE2023BSTAR 45 Other WISE-selected source in the star-forming region NGC 2023 563
KOE2068BSTAR 46 Other WISE-selected source in the star-forming region NGC 2068 602
KOEKAPAPBSTAR 47 Other WISE-selected source in the star-forming region Kappa Ori 542
TAU_STAR 52 Spitzer-selected source in the Taurus Heiles 2 molecular cloud 734

## Description

This ancillary science program obtained spectra of candidate young stellar objects (YSO) in the Orion and Taurus molecular clouds. Because this program leverages the large number of BOSS fibers to extend target selection to fainter magnitudes, it enables the discovery of very low luminosity, low mass objects.

The data provide a census of YSO into the brown dwarf regime, a measurement of the initial mass function at low masses, and a characterization of circumstellar disks as a function of stellar mass, extending previous studies to fainter magnitudes.

## Target Selection

Objects were selected mostly from WISE photometry, as well as Two Micron All Sky Survey (2MASS; Skrutskie et al. 2006) and Spitzer photometry (matching to SDSS imaging where available; Finkbeiner et al. 2004) in the Orion and Taurus regions. Objects were included to the detection limit of the WISE catalog, but those with W1 > 7 were removed to reduce contamination from luminous, very red, asymptotic giant branch stars.

There are several target classes within this program, each indicated by a separate bit in an object’s ANCILLARY_TARGET2 value. Bit values are listed in the table above and in the DR12 Bitmasks page.

The program used five plates, designed in a heterogeneous manner due to the different availability of SDSS imaging in each field and the variation in the relative number of IR-excess sources; the latter is primarily related to the age of each star formation complex, since the circumstellar disk fraction decreases with stellar age. When limited SDSS photometry was available in a field, gri magnitudes were derived from the PPMXL/USNO-B1 catalog following the inverse of the transformations tabulated in Monet et al. (2003).

Target selection for each of the five plates is described below, sorted by the four plates in Orion and then the one plate in Taurus.

### Orion Plates

Targets on the 25 Ori spectroscopic plate (7261) were WISE-detected stars within 1.5 degrees of the B3 star 25 Ori. This plate focuses on members of the young 25 Ori group and surrounding pre-main sequence stars. Targets on this plate are indicated by the target classes 25ORI_WISE and 25ORI_WISE_W3. Targets that were selected from the WISE catalog with detections in W1 and W3, with a magnitude limit of W3 < 11.65 are assigned a target class of 25ORI_WISE_W3. Sources were required to be fainter than 15 mag in SDSS filters g, r, and i, and brighter than g = 22 and i = 21.

The remaining three Orion plates (7257, 7259, 7260) covering the Kappa Ori, NGC 2023, and NGC 2068 star formation regions were created in an identical manner, and are indicated by the target classes KOEKAP_STAR, KOEKAPBSTAR, KOE2023_STAR, KOE2023BSTAR, KOE2068_STAR, and KOE2068BSTAR. For all three plates, objects in the *_STAR class are infrared-excess sources selected by W1 – W2 > 0 and SNR in W1 greater than 10. Targets indicated by the *BSTAR class are other WISE detections within those fields.

### Taurus Plate

The single Taurus spectroscopic plate (7262) consists of targets objects with Spitzer mid-infrared 8-micron and/or 24-micron excess within 1.5 degrees of the center of the Taurus Heiles 2 molecular cloud. Our sample for Taurus focuses on very-low-mass substellar objects with disks, and on edge-on disks which may have been mistaken for galaxies.

Target selection on the Taurus plate used IRAC1 – IRAC4 > 1.5 and/or IRAC1 – MIPS24 > 1.5 mag with SNR > 10 for IRAC1 and SNR > 7 for IRAC4 or MIPS24. Here IRAC1, IRAC4, and MIPS24 refer to Vega magnitudes measured through filters centered at 3.5, 8.0, and 24 microns on Spitzer.

All science objects on the Taurus plate have a target class of TAU_STAR.

## REFERENCES

Finkbeiner, D. P., et al. 2004, AJ, 128, 2577

Monet, D. G., et al. 2003, AJ, 125, 984

Skrutskie, M. F., et al. 2006, AJ, 131, 1163