28 research outputs found
Modeling Stellar Parameters for High Resolution Late-M and Early-L Dwarf SDSS/APOGEE Spectra
The Apache Point Observatory Galactic Evolution Experiment (APOGEE) of the Sloan Digital Sky Survey IV has measured high resolution (R~22,500), near-infrared (1.51-1.70 µm) spectra for nearly 100,000 stars within the Milky Way Galxy [1]. While the APOGEE experiment was originally designed to research Galactic structure by targeting bright stellar populations in the disk, we have focused attention on the lesser-studied subset of faint and low-temperature late-M and early-L dwarfs, with the objective of characterizing their chemical abundances. Using spectral sythesis routines from the Starfish package [2], we report preliminary determinations of Teff, logg, and [Fe/H] for a small sample of spectra using PHOENIX models [3]. We also compare these PHOENIX fits to low-resolution data from the SpeX Prism Library [4] fitted by BT-Settl models
Prospects for Detecting Gaps in Globular Cluster Stellar Streams in External Galaxies with the Nancy Grace Roman Space Telescope
Stellar streams form through the tidal disruption of satellite galaxies or
globular clusters orbiting a host galaxy. Globular cluster streams are exciting
since they are thin (dynamically cold) and, therefore sensitive to
perturbations from low-mass subhalos. Since the subhalo mass function differs
depending on the dark matter composition, these gaps can provide unique
constraints on dark matter models. However, current samples are limited to the
Milky Way. With its large field of view, deep imaging sensitivity, and high
angular resolution, the upcoming Nancy Grace Roman Space Telescope (Roman)
presents a unique opportunity to increase the number of observed streams and
gaps significantly. This paper presents a first exploration of the prospects
for detecting gaps in streams in M31 and other nearby galaxies with resolved
stars. We simulate the formation of gaps in a Palomar-5-like stream and
generate mock observations of these gaps with background stars in M31 and the
foreground Milky Way stellar fields. We assess Roman's ability to detect gaps
out to 10 Mpc through visual inspection and with the gap-finding tool
. We conclude that gaps of kpc in streams
that are created from subhalos of masses M are
detectable within a 2-3 Mpc volume in exposures of 1000s to 1 hour. This volume
contains galaxies, including galaxies with
luminosities L. Large samples of stream gaps in external
galaxies will open up a new era of statistical analyses of gap characteristics
in stellar streams and help constrain dark matter models.Comment: ApJ versio
Beyond the Local Volume. II. Population Scaleheights and Ages of Ultracool Dwarfs in Deep HST/WFC3 Parallel Fields
Ultracool dwarfs (UCDs) represent a significant proportion of stars in the Milky Way, and deep samples of these sources have the potential to constrain the formation history and evolution of low-mass objects in the Galaxy. Until recently, spectral samples have been limited to the local volume (d \u3c 100 pc). Here, we analyze a sample of 164 spectroscopically characterized UCDs identified by Aganze et al. in the Hubble Space Telescope (HST) WFC3 Infrared Spectroscopic Parallel Survey (WISPS) and 3D-HST. We model the observed luminosity function using population simulations to place constraints on scaleheights, vertical velocity dispersions, and population ages as a function of spectral type. Our star counts are consistent with a power-law mass function and constant star formation history for UCDs, with vertical scaleheights of 249 pc for late-M dwarfs, 153 pc for L dwarfs, and 175 pc for T dwarfs. Using spatial and velocity dispersion relations, these scaleheights correspond to disk population ages of 3.6 Gyr for late-M dwarfs, 2.1 Gyr for L dwarfs, and 2.4 Gyr for T dwarfs, which are consistent with prior simulations that predict that L-type dwarfs are on average a younger and less dispersed population. There is an additional 1–2 Gyr systematic uncertainty on these ages due to variances in age-velocity relations. We use our population simulations to predict the UCD yield in the James Webb Space Telescope PASSAGES survey, a similar and deeper survey to WISPS and 3D-HST, and find that it will produce a comparably sized UCD sample, albeit dominated by thick disk and halo sources
Beyond the Local Volume. I. Surface Densities of Ultracool Dwarfs in Deep HST/WFC3 Parallel Fields
Ultracool dwarf stars and brown dwarfs provide a unique probe of large-scale Galactic structure and evolution; however, until recently spectroscopic samples of sufficient size, depth, and fidelity have been unavailable. Here, we present the identification of 164 M7-T9 ultracool dwarfs in 0.6 deg2 of deep, low-resolution, near-infrared spectroscopic data obtained with the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) instrument as part of the WFC3 Infrared Spectroscopic Parallel Survey and the 3D-HST survey. We describe the methodology by which we isolate ultracool dwarf candidates from over 200,000 spectra, and show that selection by machine-learning classification is superior to spectral index-based methods in terms of completeness and contamination. We use the spectra to accurately determine classifications and spectrophotometric distances, the latter reaching to ∼2 kpc for L dwarfs and ∼400 pc for T dwarfs
Beyond the Local Volume II: Population Scaleheights and Ages of Ultracool Dwarfs in Deep HST/WFC3 Parallel Fields
Ultracool dwarfs represent a significant proportion of stars in the Milky
Way,and deep samples of these sources have the potential to constrain the
formation history and evolution of low-mass objects in the Galaxy. Until
recently, spectral samples have been limited to the local volume (d<100 pc).
Here, we analyze a sample of 164 spectroscopically-characterized ultracool
dwarfs identified by Aganze et al. (2022) in the Hubble Space Telescope WFC3
Infrared Spectroscopic Parallel (WISP) Survey and 3D-HST. We model the observed
luminosity function using population simulations to place constraints on
scaleheights, vertical velocity dispersions and population ages as a function
of spectral type. Our star counts are consistent with a power-law mass function
and constant star formation history for ultracool dwarfs, with vertical
scaleheights 249 pc for late M dwarfs, 153 pc for L
dwarfs, and 175 pc for T dwarfs. Using spatial and velocity
dispersion relations, these scaleheights correspond to disk population ages of
3.6 for late M dwarfs, 2.1 Gyr for L dwarfs,
and 2.4 Gyr for T dwarfs, which are consistent with prior
simulations that predict that L-type dwarfs are on average a younger and less
dispersed population. There is an additional 1-2 Gyr systematic uncertainty on
these ages due to variances in age-velocity relations. We use our population
simulations to predict the UCD yield in the JWST PASSAGES survey, a similar and
deeper survey to WISPS and 3D-HST, and find that it will produce a
comparably-sized UCD sample, albeit dominated by thick disk and halo sources.Comment: submitted to Ap
Spitzer Follow-up of Extremely Cold Brown Dwarfs Discovered by the Backyard Worlds: Planet 9 Citizen Science Project
We present Spitzer follow-up imaging of 95 candidate extremely cold brown dwarfs discovered by the Backyard Worlds: Planet 9 citizen science project, which uses visually perceived motion in multiepoch Wide-field Infrared Survey Explorer (WISE) images to identify previously unrecognized substellar neighbors to the Sun. We measure Spitzer [3.6]–[4.5] color to phototype our brown dwarf candidates, with an emphasis on pinpointing the coldest and closest Y dwarfs within our sample. The combination of WISE and Spitzer astrometry provides quantitative confirmation of the transverse motion of 75 of our discoveries. Nine of our motion-confirmed objects have best-fit linear motions larger than 1'' yr⁻¹; our fastest-moving discovery is WISEA J155349.96+693355.2 (μ ≈ 2.”15 yr⁻¹), a possible T-type subdwarf. We also report a newly discovered wide-separation (~400 au) T8 comoving companion to the white dwarf LSPM J0055+5948 (the fourth such system to be found), plus a candidate late T companion to the white dwarf LSR J0002+6357 at 5 5 projected separation (~8700 au if associated). Among our motion-confirmed targets, five have Spitzer colors most consistent with spectral type Y. Four of these five have exceptionally red Spitzer colors suggesting types of Y1 or later, adding considerably to the small sample of known objects in this especially valuable low-temperature regime. Our Y dwarf candidates begin bridging the gap between the bulk of the Y dwarf population and the coldest known brown dwarf
Spitzer Follow-up of Extremely Cold Brown Dwarfs Discovered by the Backyard Worlds: Planet 9 Citizen Science Project
We present Spitzer follow-up imaging of 95 candidate extremely cold brown
dwarfs discovered by the Backyard Worlds: Planet 9 citizen science project,
which uses visually perceived motion in multi-epoch WISE images to identify
previously unrecognized substellar neighbors to the Sun. We measure Spitzer
[3.6]-[4.5] color to phototype our brown dwarf candidates, with an emphasis on
pinpointing the coldest and closest Y dwarfs within our sample. The combination
of WISE and Spitzer astrometry provides quantitative confirmation of the
transverse motion of 75 of our discoveries. Nine of our motion-confirmed
objects have best-fit linear motions larger than 1"/yr; our fastest-moving
discovery is WISEA J155349.96+693355.2 (total motion ~2.15"/yr), a possible T
type subdwarf. We also report a newly discovered wide-separation (~400 AU) T8
comoving companion to the white dwarf LSPM J0055+5948 (the fourth such system
to be found), plus a candidate late T companion to the white dwarf LSR
J0002+6357 at 5.5' projected separation (~8,700 AU if associated). Among our
motion-confirmed targets, five have Spitzer colors most consistent with
spectral type Y. Four of these five have exceptionally red Spitzer colors
suggesting types of Y1 or later, adding considerably to the small sample of
known objects in this especially valuable low-temperature regime. Our Y dwarf
candidates begin bridging the gap between the bulk of the Y dwarf population
and the coldest known brown dwarf.Comment: accepted for publication in The Astrophysical Journa