1,440 research outputs found
The stellar population structure of the Galactic disk
The spatial structure of stellar populations with different chemical
abundances in the Milky Way contains a wealth of information on Galactic
evolution over cosmic time. We use data on 14,699 red-clump stars from the
APOGEE survey, covering 4 kpc <~ R <~ 15 kpc, to determine the structure of
mono-abundance populations (MAPs)---stars in narrow bins in [a/Fe] and
[Fe/H]---accounting for the complex effects of the APOGEE selection function
and the spatially-variable dust obscuration. We determine that all MAPs with
enhanced [a/Fe] are centrally concentrated and are well-described as
exponentials with a scale length of 2.2+/-0.2 kpc over the whole radial range
of the disk. We discover that the surface-density profiles of low-[a/Fe] MAPs
are complex: they do not monotonically decrease outwards, but rather display a
peak radius ranging from ~5 kpc to ~13 kpc at low [Fe/H]. The extensive radial
coverage of the data allows us to measure radial trends in the thickness of
each MAP. While high-[a/Fe] MAPs have constant scale heights, low-[a/Fe] MAPs
flare. We confirm, now with high-precision abundances, previous results that
each MAP contains only a single vertical scale height and that low-[Fe/H],
low-[a/Fe] and high-[Fe/H], high-[a/Fe] MAPs have intermediate (h_Z~300 to 600
pc) scale heights that smoothly bridge the traditional thin- and thick-disk
divide. That the high-[a/Fe], thick disk components do not flare is strong
evidence against their thickness being caused by radial migration. The
correspondence between the radial structure and chemical-enrichment age of
stellar populations is clear confirmation of the inside-out growth of galactic
disks. The details of these relations will constrain the variety of physical
conditions under which stars form throughout the MW disk.Comment: Code available at https://github.com/jobovy/apogee-map
CGRaBS: An All-Sky Survey of Gamma-Ray Blazar Candidates
We describe a uniform all-sky survey of bright blazars, selected primarily by
their flat radio spectra, that is designed to provide a large catalog of likely
gamma-ray AGN. The defined sample has 1625 targets with radio and X-ray
properties similar to those of the EGRET blazars, spread uniformly across the
|b| > 10 deg sky. We also report progress toward optical characterization of
the sample; of objects with known R < 23, 85% have been classified and 81% have
measured redshifts. One goal of this program is to focus attention on the most
interesting (e.g., high redshift, high luminosity, ...) sources for intensive
multiwavelength study during the observations by the Large Area Telescope (LAT)
on GLAST.Comment: 18 pages, 6 figures, 1 machine-readable table available at
http://astro.stanford.edu/CGRaBS/ ; accepted for publication in ApJ
The Blue Tip of the Stellar Locus: Measuring Reddening with the SDSS
We present measurements of reddening due to dust using the colors of stars in
the Sloan Digital Sky Survey (SDSS). We measure the color of main sequence
turn-off stars by finding the "blue tip" of the stellar locus: the prominent
blue edge in the distribution of stellar colors. The method is sensitive to
color changes of order 18, 12, 7, and 8 mmag of reddening in the colors u-g,
g-r, r-i, and i-z, respectively, in regions measuring 90' by 14'. We present
maps of the blue tip colors in each of these bands over the entire SDSS
footprint, including the new dusty southern Galactic cap data provided by the
SDSS-III. The results disfavor the best fit O'Donnell (1994) and Cardelli et
al. (1989) reddening laws, but are well described by a Fitzpatrick (1999)
reddening law with R_V = 3.1. The SFD dust map is found to trace the dust well,
but overestimates reddening by factors of 1.4, 1.0, 1.2, and 1.4 in u-g, g-r,
r-i, and i-z, largely due to the adopted reddening law. In select dusty regions
of the sky, we find evidence for problems in the SFD temperature correction. A
dust map normalization difference of 15% between the Galactic north and south
sky may be due to these dust temperature errors.Comment: 18 pages, 22 figure
Gravitational Microlensing Event Statistics for the Zwicky Transient Facility
Microlensing surveys have discovered thousands of events with almost all
events discovered within the Galactic bulge or toward the Magellanic clouds.
The Zwicky Transient Facility (ZTF), while not designed to be a microlensing
campaign, is an optical time-domain survey that observes the entire northern
sky every few nights including the Galactic plane. ZTF observes
stars in g-band and r-band and can significantly contribute to the observed
microlensing population. We predict that ZTF will observe 1100
microlensing events in three years of observing within degrees
latitude of the Galactic plane, with 500 events in the outer Galaxy
(). This yield increases to 1400 (800) events
by combining every three ZTF exposures, 1800 (900) events if ZTF
observes for a total of five years, and 2400 (1300) events for a
five year survey with post-processing image stacking. Using the microlensing
modeling software PopSyCLE, we compare the microlensing populations in the
Galactic bulge and the outer Galaxy. We also present an analysis of the
microlensing event ZTF18abhxjmj to demonstrate how to leverage these population
statistics in event modeling. ZTF will constrain Galactic structure, stellar
populations, and primordial black holes through photometric microlensing.Comment: 19 pages, 13 figures, 5 tables, accepted to ApJ (6/4/2020),
microlensing simulation catalogs available at
https://portal.nersc.gov/project/uLens/Galactic_Microlensing_Distribution
Mapping Distances Across the Perseus Molecular Cloud Using CO Observations, Stellar Photometry, and Gaia DR2 Parallax Measurements
We present a new technique to determine distances to major star-forming
regions across the Perseus Molecular Cloud, using a combination of stellar
photometry, astrometric data, and spectral-line maps.
Incorporating the Gaia DR2 parallax measurements when available, we start by
inferring the distance and reddening to stars from their Pan-STARRS1 and 2MASS
photometry, based on a technique presented in Green et al. 2014; Green et al.
2015 and implemented in their 3D "Bayestar" dust map of three-quarters of the
sky. We then refine the Green et al. technique by using the velocity slices of
a CO spectral cube as dust templates and modeling the cumulative distribution
of dust along the line of sight towards these stars as a linear combination of
the emission in the slices. Using a nested sampling algorithm, we fit these
per-star distance-reddening measurements to find the distances to the CO
velocity slices towards each star-forming region. This results in distance
estimates explicitly tied to the velocity structure of the molecular gas. We
determine distances to the B5, IC348, B1, NGC1333, L1448, and L1451
star-forming regions and find that individual clouds are located between
pc, with typical combined uncertainties of . We
find that the velocity gradient across Perseus corresponds to a distance
gradient of about 25 pc, with the eastern portion of the cloud farther away
than the western portion. We determine an average distance to the complex of
pc, about 60 pc higher than the distance derived to the western
portion of the cloud using parallax measurements of water masers associated
with young stellar objects. The method we present is not limited to the Perseus
Complex, but may be applied anywhere on the sky with adequate CO data in the
pursuit of more accurate 3D maps of molecular clouds in the solar neighborhood
and beyond.Comment: Accepted for publication in The Astrophysical Journa
Photometric Calibration of the First 1.5 Years of the Pan-STARRS1 Survey
We present a precise photometric calibration of the first 1.5 years of
science imaging from the Pan-STARRS1 survey (PS1), an ongoing optical survey of
the entire sky north of declination -30 degrees in five bands. Building on the
techniques employed by Padmanabhan et al. (2008) in the Sloan Digital Sky
Survey (SDSS), we use repeat PS1 observations of stars to perform the relative
calibration of PS1 in each of its five bands, solving simultaneously for the
system throughput, the atmospheric transparency, and the large-scale detector
flat field. Both internal consistency tests and comparison against the SDSS
indicate that we achieve relative precision of <10 mmag in g, r, and i_P1, and
~10 mmag in z and y_P1. The spatial structure of the differences with the SDSS
indicates that errors in both the PS1 and SDSS photometric calibration
contribute similarly to the differences. The analysis suggests that both the
PS1 system and the Haleakala site will enable <1% photometry over much of the
sky.Comment: 16 pages, 9 figures (arxiv version updated to published version
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