173 research outputs found
Faint NUV/FUV Standards from Swift/UVOT, GALEX and SDSS Photometry
At present, the precision of deep ultraviolet photometry is somewhat limited
by the dearth of faint ultraviolet standard stars. In an effort to improve this
situation, we present a uniform catalog of eleven new faint (u sim17)
ultraviolet standard stars. High-precision photometry of these stars has been
taken from the Sloan Digital Sky Survey and Galaxy Evolution Explorer and
combined with new data from the Swift Ultraviolet Optical Telescope to provide
precise photometric measures extending from the Near Infrared to the Far
Ultraviolet. These stars were chosen because they are known to be hot (20,000 <
T_eff < 50,000 K) DA white dwarfs with published Sloan spectra that should be
photometrically stable. This careful selection allows us to compare the
combined photometry and Sloan spectroscopy to models of pure hydrogen
atmospheres to both constrain the underlying properties of the white dwarfs and
test the ability of white dwarf models to predict the photometric measures. We
find that the photometry provides good constraint on white dwarf temperatures,
which demonstrates the ability of Swift/UVOT to investigate the properties of
hot luminous stars. We further find that the models reproduce the photometric
measures in all eleven passbands to within their systematic uncertainties.
Within the limits of our photometry, we find the standard stars to be
photometrically stable. This success indicates that the models can be used to
calibrate additional filters to our standard system, permitting easier
comparison of photometry from heterogeneous sources. The largest source of
uncertainty in the model fitting is the uncertainty in the foreground reddening
curve, a problem that is especially acute in the UV.Comment: Accepted for publication in Astrophysical Journal. 31 pages, 13
figures, electronic tables available from ApJ or on reques
The D/H Ratio in Interstellar Gas Towards G191-B2B
We reinvestigate the question of spatial variation of the local D/H
abundance, using both archival GHRS spectra, and new echelle spectra of
G191-B2B obtained with the Space Telescope Imaging Spectrograph (STIS) aboard
HST. Our analysis uses stratified line-blanketed non-LTE model atmosphere
calculations to determine the shape of the intrinsic WD Lyman-alpha profile and
estimate the WD photospheric contamination of the interstellar lines. Although
three velocity components were reported previously towards G191-B2B, we detect
only two velocity components. The first component is at V(hel) ~ 8.6 km/s and
the second at V(hel) ~ 19.3 km/s, which we identify with the Local Interstellar
Cloud (LIC). From the STIS data we derive D/H = 1.60(+0.39,-0.27)X10^-5 for the
LIC component, and D/H > 1.26X10^-5 for the 8.6 km/s component (uncertainties
denote 2-sigma or 95% confidence limits). The STIS data provide no evidence for
local or component-to-component variation in the D/H ratio. Despite using two
velocity components for the profile fitting and using a more physically
realistic WD Lyman-alpha profile for G191-B2B, our re-analysis of the GHRS data
indicates a component-to-component variation as well as a variation of the D/H
ratio in the LISM, neither of which are supported by the newer STIS data. We
believe the most probable cause for this difference is the characterization of
the background due to scattered light in the GHRS and STIS spectrographs. The
two-dimensional MAMA detectors of STIS measure both the spatial and wavelength
dependences of scattered light, allowing more accurate scattered light
corrections than was possible with GHRS.Comment: Accepted for publication in Astrophysical Journal Letters. 10 pages +
3 figures. (Abstract is abridged.
Photometry and spectroscopy of faint candidate spectrophotometric standard DA white dwarfs
We present precise photometry and spectroscopy for 23 candidate
spectrophotometric standard white dwarfs. The selected stars are distributed in
the Northern hemisphere and around the celestial equators and are all fainter
than r ~ 16.5 mag. This network of stars, when established as standards,
together with the three Hubble Space Telescope primary CALSPEC white dwarfs,
will provide a set of spectrophotometric standards to directly calibrate data
products to better than 1%. These new faint standard white dwarfs will have
enough signal-to-noise ratio in future deep photometric surveys and facilities
to be measured accurately while still avoiding saturation in such surveys. They
will also fall within the dynamic range of large telescopes and their
instruments for the foreseeable future. This paper discusses the provenance of
the observational data for our candidate standard stars. The comparison with
models, reconciliation with reddening, and the consequent derivation of the
full spectral energy density distributions for each of them is reserved for a
subsequent paper.Comment: 31 pages, 17 figures, 10 tables, ApJ in press (accepted on December
23rd, 2018
The Formation Rate, Mass and Luminosity Functions of DA White Dwarfs from the Palomar Green Survey
Spectrophotometric observations at high signal-to-noise ratio were obtained
of a complete sample of 347 DA white dwarfs from the Palomar Green (PG) Survey.
Fits of observed Balmer lines to synthetic spectra calculated from
pure-hydrogen model atmospheres were used to obtain robust values of Teff, log
g, masses, radii, and cooling ages. The luminosity function of the sample,
weighted by 1/Vmax, was obtained and compared with other determinations. The
mass distribution of the white dwarfs is derived, after important corrections
for the radii of the white dwarfs in this magnitude-limited survey and for the
cooling time scales. The formation rate of DA white dwarfs from the PG is
estimated to be 0.6x10^(-12) pc^(-3) yr^(-1). Comparison with predictions from
a theoretical study of the white dwarf formation rate for single stars
indicates that >80% of the high mass component requires a different origin,
presumably mergers of lower mass double degenerate stars. In order to estimate
the recent formation rate of all white dwarfs in the local Galactic disk,
corrections for incompleteness of the PG, addition of the DB-DO white dwarfs,
and allowance for stars hidden by luminous binary companions had to be applied
to enhance the rate. An overall formation rate of white dwarfs recently in the
local Galactic disk of 1.15+/-0.25x10^(-12) pc^(-3) yr^(-1) is obtained. Two
recent studies of samples of nearby Galactic planetary nebulae lead to
estimates around twice as high. Difficulties in reconciling these
determinations are discussed.Comment: 73 pages, 18 figures, accepted for publication in the ApJ Supplemen
Sub-percent Photometry: Faint DA White Dwarf Spectophotometric Standards for Astrophysical Observatories
We have established a network of 19 faint (16.5 mag 19 mag) northern
and equatorial DA white dwarfs as spectrophotometric standards for present and
future wide-field observatories. Our analysis infers SED models for the stars
that are tied to the three CALSPEC primary standards. Our SED models are
consistent with panchromatic Hubble Space Telescope () photometry to
better than 1%. The excellent agreement between observations and models
validates the use of non-local-thermodynamic-equilibrium (NLTE) DA white dwarf
atmospheres extinguished by interstellar dust as accurate spectrophotometric
references. Our standards are accessible from both hemispheres and suitable for
ground and space-based observatories covering the ultraviolet to the near
infrared. The high-precision of these faint sources make our network of
standards ideally suited for any experiment that has very stringent
requirements on flux calibration, such as studies of dark energy using the
Large Synoptic Survey Telescope (LSST) and the Wide-Field Infrared Survey
Telescope ().Comment: 46 pages, 23 figures, 8 tables, accepted for publication in ApJ
Multiwavelength Observations of the Hot DB Star PG 0112+104
We present a comprehensive multiwavelength analysis of the hot DB white dwarf
PG 0112+104. Our analysis relies on newly-acquired FUSE observations, on
medium-resolution FOS and GHRS data, on archival high-resolution GHRS
observations, on optical spectrophotometry both in the blue and around Halpha,
as well as on time-resolved photometry. From the optical data, we derive a
self-consistent effective temperature of 31,300+-500 K, a surface gravity of
log g = 7.8 +- 0.1 (M=0.52 Msun), and a hydrogen abundance of log N(H)/N(He) <
-4.0. The FUSE spectra reveal the presence of CII and CIII lines that
complement the previous detection of CII transitions with the GHRS. The
improved carbon abundance in this hot object is log N(C)/N(He) = -6.15 +- 0.23.
No photospheric features associated with other heavy elements are detected. We
reconsider the role of PG 0112+104 in the definition of the blue edge of the
V777 Her instability strip in light of our high-speed photometry, and contrast
our results with those of previous observations carried out at the McDonald
Observatory.Comment: 10 pages in emulateapj, 9 figures, accepted for publication in Ap
Modeling of the Lyman gamma satellites in FUSE spectra of DA white dwarfs
We present new theoretical calculations of the line profile of Lyman gamma
that include transitions in which a photon is absorbed by a neutral hydrogen
atom while it interacts with a proton. Models show that two absorption features
located near 992 A and 996 A are due to H-H^+ collisions. These quasi-molecular
satellites are similar to those that were identified in the wings of Lyman
alpha and Lyman beta lines of hydrogen-rich white dwarfs. We compute synthetic
spectra that take account of these new theoretical profiles and compare them to
the spectra of four DA white dwarfs that were observed with FUSE. The models
predict the absorption features that are observed in the wing of Lyman gamma
near 995 A, and confirm that these features are quasi-molecular satellites.Comment: 5 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
Discovery of the Coldest Imaged Companion of a Sun-Like Star
We present the discovery of a brown dwarf or possible planet at a projected
separation of 1.9" = 29 AU around the star GJ 758, placing it between the
separations at which substellar companions are expected to form by core
accretion (~5 AU) or direct gravitational collapse (typically >100 AU). The
object was detected by direct imaging of its thermal glow with Subaru/HiCIAO.
At 10-40 times the mass of Jupiter and a temperature of 550-640 K, GJ 758 B
constitutes one of the few known T-type companions, and the coldest ever to be
imaged in thermal light around a Sun-like star. Its orbit is likely eccentric
and of a size comparable to Pluto's orbit, possibly as a result of
gravitational scattering or outward migration. A candidate second companion is
detected at 1.2" at one epoch.Comment: 5 pages, 3 figures, 2 tables. Accepted for publication in ApJ Letter
All-Sky Faint DA White Dwarf Spectrophotometric Standards for Astrophysical Observatories: The Complete Sample
Hot DA white dwarfs have fully radiative pure hydrogen atmospheres that are
the least complicated to model. Pulsationally stable, they are fully
characterized by their effective temperature Teff, and surface gravity log g,
which can be deduced from their optical spectra and used in model atmospheres
to predict their spectral energy distribution (SED). Based on this, three
bright DAWDs have defined the spectrophotometric flux scale of the CALSPEC
system of HST. In this paper we add 32 new fainter (16.5 < V < 19.5) DAWDs
spread over the whole sky and within the dynamic range of large telescopes.
Using ground based spectra and panchromatic photometry with HST/WFC3, a new
hierarchical analysis process demonstrates consistency between model and
observed fluxes above the terrestrial atmosphere to < 0.004 mag rms from 2700
{\AA} to 7750 {\AA} and to 0.008 mag rms at 1.6{\mu}m for the total set of 35
DAWDs. These DAWDs are thus established as spectrophotometric standards with
unprecedented accuracy from the near ultraviolet to the near-infrared, suitable
for both ground and space based observatories. They are embedded in existing
surveys like SDSS, PanSTARRS and GAIA, and will be naturally included in the
LSST survey by Rubin Observatory. With additional data and analysis to extend
the validity of their SEDs further into the IR, these spectrophotometric
standard stars could be used for JWST, as well as for the Roman and Euclid
observatories.Comment: Accepted for publication in Astrophysical Journal. Corrected error in
Table 10 and associated Fig 7 in which RP and BP values for the 3 CALSPEC
standards had been transpose
- …