The Transient Universe through Stripe 82


Photo of Cullen Blake
Cullen Blake
University of Pennsylvania


Spectra of a variety of variable objects in a 220-square-degree section of the repeated imaging of SDSS stripe 82

Finding Targets

An object whose ANCILLARY_TARGET1 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 Spectra Unique Primary Objects
AMC 0 Candidate Am CVn variables 22 20
FLARE1 1 Flaring M stars (year 1 targets) 30 28
FLARE2 2 Flaring M stars (year 2 targets) 59 42
HPM 3 High Proper Motion stars 75 72
LOW_MET 4 Low-metallicity M dwarfs 53 50
VARS 5 Variables outside the stellar and quasar loci 165 136
MTEMP 63 Template M-stars observed as a comparison sample 0 0


The repeat imaging in SDSS’s Stripe 82 allows identification of transient and variable phenomena of all sorts (for example Anderson et al. 2008; Blake et al. 2008; Becker et al. 2008; Kowalski et al. 2009; Bhatti et al. 2010; Becker et al. 2011; Sako et al. 2011). In this program, several classes of variable point sources and high-proper-motion stars discovered in Stripe 82 photometry were targeted for follow-up spectroscopy with the BOSS spectrograph. Targeted objects include flaring M stars (Kowalski et al. 2009), faint high proper motion stars (Scholz et al. 2009; Schmidt et al. 2010), candidate low-metallicity M dwarfs, and diverse samples of variable stars (Roelofs et al. 2007; Anderson et al. 2008; Blake et al. 2008).

The target selection criteria for each class of transient objects is described below.

Target Selection

The target selection for this program is complicated. Each of the six sub-programs used a different target selection algorithm. The paragraphs below describe the algorithm for each sub-program.

Candidate Am CVn Stars

Consists of ten candidate Am CVn stars selected based on their variability and colors ( (uPSF – gPSF) > 0.4 and (gPSF – rPSF) > 0 ).

To identify candidate AM CVn stars in the Stripe 82 footprint, this ancillary target program used variability indices developed by Stetson (1996), which take advantage of simultaneous multi-band observations of stars with AM CVn-like colors. The color selection criteria were defined based on the colors of known AM CVn stars, but do allowed for objects bluer than any known AM CVn system.

The addition of variability information to the color-selected sample has allows a subset of variable star candidates to be defined; this subset should be more likely to contain genuine AM CVn stars. Quasars and Active Galactic Nuclei (AGN) are the dominant confusing variable sources in this region of color-space.

Flare Stars

Consists of about 200 flaring M stars selected from the Bramich et al. (2008) and Ivezić et al. (2007) catalogs with gPSF < 21.4, iPSF < 19, 0.3 < (iPSF – zPSF) < 1.3, and exhibiting a flare event in the Stripe 82 imaging data with amplitude Δu > 1 mag (Kowalski et al. 2009).

The FLARE1 and FLARE2 targets were selected using slightly different methodologies for estimating the quiescent u-band magnitudes of the M stars.

High Proper Motion Stars

Consists of about 100 high-proper-motion stars selected from the catalogs of Bramich et al. (2008) and Ivezić et al. (2007) with an emphasis on faint objects with high proper motions (μ > 0.1 mas/yr).

The goal of this target selection sub-program was to identify nearby low-mass stars and white dwarfs. Candidate nearby low-mass stars include faint stars (19 < zPSF < 20) with (iPSF – zPSF) > 1.5, including objects with photometric detections in z-band only. Candidate nearby white dwarfs include stars with (gPSF – rPSF) ~ 0 and gPSF > 19.

Low-Metallicity M Dwarfs

Approximately 70 candidate low-metallicity M stars (flag LOW_MET) were also targeted based on their colors being slightly outside the low-mass star stellar locus defined in West et al. (2005).

Other Variable Objects

Consists of ~ 200 variables with gPSF < 21.5, (gPSF – rPSF) > -0.5, and RMS variability in g-band > 0.1 mag. These objects were selected to lie outside the stellar and quasar loci in (gPSF – rPSF) – (rPSF – iPSF) color-color space as defined in Fan et al. (1999).


As part of this ancillary program, a number of spectral templates were observed as well. These include a random sample of approximately 100 M stars, designated MTEMP, selected to span the spectra range M0 to M8 following the color criteria outlined in West et al. (2005) (17.5 < iPSF < 18.5, (rPSF – iPSF) > 0.5, (iPSF – zPSF) > 0.3).


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