34 research outputs found
The Ultraviolet Sky: An Overview from the GALEX Surveys
The Galaxy Evolution Explorer (GALEX) has performed the first surveys of the
sky in the Ultraviolet (UV). Its legacy is an unprecedented database with more
than 200 million source measurements in far-UV (FUV) and near-UV (NUV), as well
as wide-field imaging of extended objects, filling an important gap in our view
of the sky across the electromagnetic spectrum. The UV surveys offer unique
sensitivity for identifying and studying selected classes of astrophysical
objects, both stellar and extra-galactic. We examine the overall content and
distribution of UV sources over the sky, and with magnitude and color. For this
purpose, we have constructed final catalogs of UV sources with homogeneous
quality, eliminating duplicate measurements of the same source. Such catalogs
can facilitate a variety of investigations on UV-selected samples, as well as
planning of observations with future missions.
We describe the criteria used to build the catalogs, their coverage and
completeness. We included observations in which both the far-UV and near-UV
detectors were exposed; 28,707 fields from the All-Sky Imaging survey (AIS)
cover a unique area of 22,080 square degrees (after we restrict the catalogs to
the central 1-degree diameter of the field), with a typical depth of about
20/21 mag (FUV/NUV, in the AB mag system), and 3,008 fields from the
Medium-depth Imaging Survey (MIS) cover a total of 2,251 square degrees at a
depth of about 22.7mag. The catalogs contain about 71 and 16.6 million sources
respectively. The density of hot stars reflects the Galactic structure, and the
number counts of both Galactic and extra-galactic sources are modulated by the
Milky Way dust extinction, to which the UV data are very sensitive.Comment: J. Adv. Space Res. (2013), Full resolution figures can be found in
the original published article (open access) at :
http://www.sciencedirect.com/science/article/pii/S0273117713004742 or from
http://dolomiti.pha.jhu.edu/publgoto.html ; catalogs are posted on MAS
A Study of the Efficiency of Spatial Indexing Methods Applied to Large Astronomical Databases
We report the results of a study to compare the performance of two common database indexing methods, HTM and HEALPix, on Solaris and Windows database servers installed with PostgreSQL, and a Windows Server installed with MS SQL Server. The indexing was applied to the 2MASS All-Sky Catalog and to the Hubble Source Catalog, which approximate the diversity of catalogs common in astronomy. On each server, the study compared indexing performance by submitting 1 million queries at each index level with random sky positions and random cone search radius, which was computed on a logarithmic scale between 1 arcsec and 1 degree, and measuring the time to complete the query and write the output. These simulated queries, intended to model realistic use patterns, were run in a uniform way on many combinations of indexing method and indexing depth. The query times in all simulations are strongly I/O-bound and are linear with number of records returned for large numbers of sources. There are, however, considerable differences between simulations, which reveal that hardware I/O throughput is a more important factor in managing the performance of a DBMS than the choice of indexing scheme. The choice of index itself is relatively unimportant: for comparable index levels, the performance is consistent within the scatter of the timings. At small index levels (large cells; e.g. level 4; cell size 3.7 deg), there is large scatter in the timings because of wide variations in the number of sources found in the cells. At larger index levels, performance improves and scatter decreases, but the improvement at level 8 (14 arcmin) and higher is masked to some extent in the timing scatter caused by the range of query sizes. At very high levels (20; 0.0004 arsec), the granularity of the cells becomes so high that a large number of extraneous empty cells begin to degrade performance
A Study of the Efficiency of Spatial Indexing Methods Applied to Large Astronomical Databases
We report the results of a study to compare the performance of two common database indexing methods, HTM and HEALPix, on Solaris and Windows database servers installed with PostgreSQL, and a Windows Server installed with MS SQL Server. The indexing was applied to the 2MASS All-Sky Catalog and to the Hubble Source Catalog, which approximate the diversity of catalogs common in astronomy. On each server, the study compared indexing performance by submitting 1 million queries at each index level with random sky positions and random cone search radius, which was computed on a logarithmic scale between 1 arcsec and 1 degree, and measuring the time to complete the query and write the output. These simulated queries, intended to model realistic use patterns, were run in a uniform way on many combinations of indexing method and indexing depth. The query times in all simulations are strongly I/O-bound and are linear with number of records returned for large numbers of sources. There are, however, considerable differences between simulations, which reveal that hardware I/O throughput is a more important factor in managing the performance of a DBMS than the choice of indexing scheme. The choice of index itself is relatively unimportant: for comparable index levels, the performance is consistent within the scatter of the timings. At small index levels (large cells; e.g. level 4; cell size 3.7 deg), there is large scatter in the timings because of wide variations in the number of sources found in the cells. At larger index levels, performance improves and scatter decreases, but the improvement at level 8 (14 arcmin) and higher is masked to some extent in the timing scatter caused by the range of query sizes. At very high levels (20; 0.0004 arsec), the granularity of the cells becomes so high that a large number of extraneous empty cells begin to degrade performance
The GALEX View of "Boyajian's Star" (KIC 8462852)
The enigmatic star KIC 8462852, informally known as "Boyajian's Star", has
exhibited unexplained variability from both short timescale (days) dimming
events, and years-long fading in the Kepler mission. No single physical
mechanism has successfully explained these observations to date. Here we
investigate the ultraviolet variability of KIC 8462852 on a range of timescales
using data from the GALEX mission that occurred contemporaneously with the
Kepler mission. The wide wavelength baseline between the Kepler and GALEX data
provides a unique constraint on the nature of the variability. Using 1600
seconds of photon-counting data from four GALEX visits spread over 70 days in
2011, we find no coherent NUV variability in the system on 10-100 second or
months timescales. Comparing the integrated flux from these 2011 visits to the
2012 NUV flux published in the GALEX-CAUSE Kepler survey, we find a 3% decrease
in brightness for KIC 8462852. We find this level of variability is
significant, but not necessarily unusual for stars of similar spectral type in
the GALEX data. This decrease coincides with the secular optical fading
reported by Montet & Simon (2016). We find the multi-wavelength variability is
somewhat inconsistent with typical interstellar dust absorption, but instead
favors a R = 5.0 0.9 reddening law potentially from circumstellar
dust.Comment: 8 pages, 4 figures, ApJ Accepte
Spitzer View of Massive Star Formation in the Tidally Stripped Magellanic Bridge
The Magellanic Bridge is the nearest low-metallicity, tidally stripped
environment, offering a unique high-resolution view of physical conditions in
merging and forming galaxies. In this paper we present analysis of candidate
massive young stellar objects (YSOs), i.e., {\it in situ, current} massive star
formation (MSF) in the Bridge using {\it Spitzer} mid-IR and complementary
optical and near-IR photometry. While we definitely find YSOs in the Bridge,
the most massive are , found in the Large
Magellanic Cloud (LMC). The intensity of MSF in the Bridge also appears
decreasing, as the most massive YSOs are less massive than those formed in the
past. To investigate environmental effects on MSF, we have compared properties
of massive YSOs in the Bridge to those in the LMC. First, YSOs in the Bridge
are apparently less embedded than in the LMC: 81% of Bridge YSOs show optical
counterparts, compared to only 56% of LMC sources with the same range of mass,
circumstellar dust mass, and line-of-sight extinction. Circumstellar envelopes
are evidently more porous or clumpy in the Bridge's low-metallicity
environment. Second, we have used whole samples of YSOs in the LMC and the
Bridge to estimate the probability of finding YSOs at a given \hi\ column
density, N(HI). We found that the LMC has higher probability than
the Bridge for N(HI) cm, but the trend reverses at
lower N(HI). Investigating whether this lower efficiency relative to HI is due
to less efficient molecular cloud formation, or less efficient cloud collapse,
or both, will require sensitive molecular gas observations.Comment: 41 pages, 20 figures, 6 tables; accepted for publication in ApJ;
several figures are in low resolution due to the size limit here and a high
resolution version can be downloaded via
http://www.astro.virginia.edu/~cc5ye/ms_bridge20140215.pd
Dust Production and Mass Loss in the Galactic Globular Cluster NGC 362
We investigate dust production and stellar mass loss in the Galactic globular
cluster NGC 362. Due to its close proximity to the Small Magellanic Cloud
(SMC), NGC 362 was imaged with the IRAC and MIPS cameras onboard the Spitzer
Space Telescope as part of the Surveying the Agents of Galaxy Evolution
(SAGE-SMC) Spitzer Legacy program. We detect several cluster members near the
tip of the Red Giant Branch that exhibit infrared excesses indicative of
circumstellar dust and find that dust is not present in measurable quantities
in stars below the tip of the Red Giant Branch. We modeled the spectral energy
distribution (SED) of the stars with the strongest IR excess and find a total
cluster dust mass-loss rate of 3.0(+2.0/-1.2) x 10^-9 solar masses per year,
corresponding to a gas mass-loss rate of 8.6(+5.6/-3.4) x 10^-6 solar masses
per year, assuming [Fe/H] = -1.16. This mass loss is in addition to any
dust-less mass loss that is certainly occurring within the cluster. The two
most extreme stars, variables V2 and V16, contribute up to 45% of the total
cluster dust-traced mass loss. The SEDs of the more moderate stars indicate the
presence of silicate dust, as expected for low-mass, low-metallicity stars.
Surprisingly, the SED shapes of the stars with the strongest mass-loss rates
appear to require the presence of amorphous carbon dust, possibly in
combination with silicate dust, despite their oxygen-rich nature. These results
corroborate our previous findings in omega Centauri.Comment: 13 pages, 11 figures. Accepted to Ap
The Revised TESS Input Catalog and Candidate Target List
We describe the catalogs assembled and the algorithms used to populate the
revised TESS Input Catalog (TIC), based on the incorporation of the Gaia second
data release. We also describe a revised ranking system for prioritizing stars
for 2-minute cadence observations, and assemble a revised Candidate Target List
(CTL) using that ranking. The TIC is available on the Mikulski Archive for
Space Telescopes (MAST) server, and an enhanced CTL is available through the
Filtergraph data visualization portal system at the URL
http://filtergraph.vanderbilt.edu/tess_ctl.Comment: 30 pages, 16 figures, submitted to AAS Journals; provided to the
community in advance of publication in conjunction with public release of the
TIC/CTL on 28 May 201