79 research outputs found
The Spitzer Atlas of Stellar Spectra
We present the Spitzer Atlas of Stellar Spectra (SASS), which includes 159
stellar spectra (5 to 32 mic; R~100) taken with the Infrared Spectrograph on
the Spitzer Space Telescope. This Atlas gathers representative spectra of a
broad section of the Hertzsprung-Russell diagram, intended to serve as a
general stellar spectral reference in the mid-infrared. It includes stars from
all luminosity classes, as well as Wolf-Rayet (WR) objects. Furthermore, it
includes some objects of intrinsic interest, like blue stragglers and certain
pulsating variables. All the spectra have been uniformly reduced, and all are
available online. For dwarfs and giants, the spectra of early-type objects are
relatively featureless, dominated by Hydrogen lines around A spectral types.
Besides these, the most noticeable photospheric features correspond to water
vapor and silicon monoxide in late-type objects and methane and ammonia
features at the latest spectral types. Most supergiant spectra in the Atlas
present evidence of circumstellar gas. The sample includes five M supergiant
spectra, which show strong dust excesses and in some cases PAH features.
Sequences of WR stars present the well-known pattern of lines of HeI and HeII,
as well as forbidden lines of ionized metals. The characteristic flat-top shape
of the [Ne III] line is evident even at these low spectral resolutions. Several
Luminous Blue Variables and other transition stars are present in the Atlas and
show very diverse spectra, dominated by circumstellar gas and dust features. We
show that the [8]-[24] Spitzer colors (IRAC and MIPS) are poor predictors of
spectral type for most luminosity classes.Comment: Accepted by ApJS; Atlas contents available from:
http://web.ipac.caltech.edu/staff/ardila/Atlas/index.html;
http://irsa.ipac.caltech.edu/data/SPITZER/SASS/; 70 PDF pages, including
figure
Herschel Observations of Debris Disks from WISE
The \Vide Field Infrared Survey Explorer (WISE) has just completed a sensitive all-sky survey in photometric bands at 3.4, 4.6,12 and 22 microns. We report on a study of main sequence Hipparcos and Tycho catalog stars within 120 pc with WISE 22 micron emission in excess of photospheric levels. This warm excess emission traces material in the circumstellar region likely to host terrestrial planets and is preferentially found in young systems with ages < 1 Gyr. Nearly a hundred of the WISE new warm debris disk candidates detected among FGK stars are being observed by Herschel/PACS to characterize circumstellar dust. Preliminary results indicate 70 micron detection rates in excess of 80% for these targets, suggesting that most of these systems have both warm and cool dust in analogy to our asteroid and Kuiper belts. In this contribution, we will discuss the WISE debris disk survey and latest results from Herschel observations of these sources
The Low Velocity Wind from the Circumstellar Matter Around the B9V Star sigma Herculis
We have obtained FUSE spectra of sigma Her, a nearby binary system, with a
main sequence primary, that has a Vega-like infrared excess. We observe
absorption in the excited fine structure lines C II* at 1037 A, N II* at 1085
A, and N II** at 1086 A that are blueshifted by as much as ~30 km/sec with
respect to the star. Since these features are considerably narrower than the
stellar lines and broader than interstellar features, the C II and N II are
circumstellar. We suggest that there is a radiatively driven wind, arising from
the circumstellar matter, rather than accretion as occurs around beta Pic,
because of sigma Her's high luminosity. Assuming that the gas is liberated by
collisions between parent bodies at 20 AU, the approximate distance at which
blackbody grains are in radiative equilibrium with the star and at which 3-body
orbits become unstable, we infer dM/dt ~ 6 * 10^-12 M_{sun}/yr. This wind
depletes the minimum mass of parent bodies in less than the estimated age of
the system.Comment: 6 pages, 3 figures, ApJ in pres
New Debris Disk Candidates Around 49 Nearby Stars
We present 49 new candidate debris disks that were detected around nearby stars with the Spitzer Space Telescope using the Multiband Imaging Photometer (MIPS) at 24 μm (MIPS24) and 70 μm (MIPS70). The survey sample was composed of stars within 25 pc of the Sun that were not previously observed by any other MIPS survey. Only stars with V < 9 were selected, corresponding to spectral types earlier than M0. MIPS24 integration times were chosen to detect the stellar photosphere at 10σ levels or better. MIPS70 observations were designed to detect excess infrared emission from any star in the MIPS70 sample with a disk as luminous at that around epsilon Eridani. The resulting sample included over 436 nearby stars that were observed with both MIPS24 and MIPS70, plus an additional 198 observed only with MIPS24. Debris disk candidates were defined as targets where excess emission was detected at 3σ levels or greater, and the ratio of observed flux density to expected photosphere emission was three standard deviations or more above the mean value for the sample. The detection rate implied by the resulting 29 MIPS24 candidates is 4.6%. A detection rate of 4.8% is implied by 21 MIPS70 candidates. The distribution of spectral types for stars identified as candidates resembles that of the general sample and yields strong evidence that debris-disk occurrence does not decrease for K dwarfs. Modeling of non-uniform sensitivity in the sample is required to interpret quantitative estimates of the overall detection frequency and will be presented in a future work
Radii of 88 M subdwarfs and updated radius relations for low-metallicity M-dwarf stars
M subdwarfs are low-metallicity M dwarfs that typically inhabit the halo population of the Galaxy. Metallicity controls the opacity of stellar atmospheres; in metal-poor stars, hydrostatic equilibrium is reached at a smaller radius, leading to smaller radii for a given effective temperature. We compile a sample of 88 stars that span spectral classes K7 to M6 and include stars with metallicity classes from solar-metallicity dwarf stars to the lowest metallicity ultra subdwarfs to test how metallicity changes the stellar radius. We fit models to Palomar Double Spectrograph (DBSP) optical spectra to derive effective temperatures (T_ eff) and we measure bolometric luminosities (L_ bol) by combining broad wavelength-coverage photometry with Gaia parallaxes. Radii are then computed by combining the T_ eff and L_ bol using the Stefan–Boltzman law. We find that for a given temperature, ultra subdwarfs can be as much as five times smaller than their solar-metallicity counterparts. We present color-radius and color-surface brightness relations that extend down to [Fe/H] of −2.0 dex, in order to aid the radius determination of M subdwarfs, which will be especially important for the WFIRST exoplanetary microlensing survey.Published versio
Discovery of the Young L Dwarf WISE J174102.78–464225.5
We report the discovery of the L dwarf WISE J174102.78–464225.5, which was discovered as part of a search for nearby L dwarfs using the Wide-field Infrared Survey Explorer (WISE). The distinct triangular peak of the H-band portion of its near-infrared spectrum and its red near-infrared colors (J – K_S = 2.35 ± 0.08 mag) are indicative of a young age. Via comparison to spectral standards and other red L dwarfs, we estimate a near-infrared spectral type of L7 ± 2 (pec). From a comparison to spectral and low-mass evolutionary models, we determine self-consistent effective temperature, log g, age, and mass values of 1450 ± 100 K, 4.0 ± 0.25 (cm s^(–2)), 10-100 Myr, and 4-21 M_(Jup), respectively. With an estimated distance of 10-30 pc, we explore the possibility that WISE J174102.78–464225.5 belongs to one of the young nearby moving groups via a kinematic analysis and we find potential membership in the β Pictoris or AB Doradus associations. A trigonometric parallax measurement and a precise radial velocity can help to secure its membership in either of these groups
The AllWISE Motion Survey, Part 2
We use the AllWISE Data Release to continue our search for WISE-detected
motions. In this paper, we publish another 27,846 motion objects, bringing the
total number to 48,000 when objects found during our original AllWISE motion
survey are included. We use this list, along with the lists of confirmed
WISE-based motion objects from the recent papers by Luhman and by Schneider et
al. and candidate motion objects from the recent paper by Gagne et al. to
search for widely separated, common-proper-motion systems. We identify 1,039
such candidate systems. All 48,000 objects are further analyzed using
color-color and color-mag plots to provide possible characterizations prior to
spectroscopic follow-up. We present spectra of 172 of these, supplemented with
new spectra of 23 comparison objects from the literature, and provide
classifications and physical interpretations of interesting sources. Highlights
include: (1) the identification of three G/K dwarfs that can be used as
standard candles to study clumpiness and grain size in nearby molecular clouds
because these objects are currently moving behind the clouds, (2) the
confirmation/discovery of several M, L, and T dwarfs and one white dwarf whose
spectrophotometric distance estimates place them 5-20 pc from the Sun, (3) the
suggestion that the Na 'D' line be used as a diagnostic tool for interpreting
and classifying metal-poor late-M and L dwarfs, (4) the recognition of a triple
system including a carbon dwarf and late-M subdwarf, for which model fits of
the late-M subdwarf (giving [Fe/H] ~ -1.0) provide a measured metallicity for
the carbon star, and (5) a possible 24-pc-distant K5 dwarf + peculiar red L5
system with an apparent physical separation of 0.1 pc.Comment: 62 pages with 80 figures, accepted for publication in The
Astrophysical Journal Supplement Series, 23 Mar 2016; second version fixes a
few small typos and corrects the footnotes for Table
Circumstellar material in the Vega inner system revealed by CHARA/FLUOR
Only a handful of debris disks have been imaged up to now. Due to the need
for high dynamic range and high angular resolution, very little is known about
the inner planetary region, where small amounts of warm dust are expected to be
found. We investigate the close neighbourhood of Vega with the help of infrared
stellar interferometry and estimate the integrated K-band flux originating from
the central 8 AU of the debris disk. We performed precise visibility
measurements at both short (~30 m) and long (~150 m) baselines with the FLUOR
beam-combiner installed at the CHARA Array (Mt Wilson, California) in order to
separately resolve the emissions from the extended debris disk (short
baselines) and from the stellar photosphere (long baselines). After revising
Vega's K-band angular diameter (3.202+/-0.005 mas), we show that a significant
deficit in squared visibility (1.88+/-0.34%) is detected at short baselines
with respect to the best-fit uniform disk stellar model. This deficit can be
either attributed to the presence of a low-mass stellar companion around Vega,
or as the signature of the thermal and scattered emissions from the debris
disk. We show that the presence of a close companion is highly unlikely, as
well as other possible perturbations (stellar morphology, calibration), and
deduce that we have most probably detected the presence of dust in the close
neighbourhood of Vega. The resulting flux ratio between the stellar photosphere
and the debris disk amounts to 1.29+/-0.19% within the FLUOR field-of-view
(~7.8 AU). Finally, we complement our K-band study with archival photometric
and interferometric data in order to evaluate the main physical properties of
the inner dust disk. The inferred properties suggest that the Vega system could
be currently undergoing major dynamical perturbations.Comment: A&A, accepted -- Press release available at
http://www2.cnrs.fr/presse/communique/848.ht
Structure and Colors of Diffuse Emission in the Spitzer Galactic First Look Survey
We investigate the density structure of the interstellar medium using new
high-resolution maps of the 8 micron, 24 micron, and 70 micron surface
brightness towards a molecular cloud in the Gum Nebula, made as part of the
Spitzer Space Telescope Galactic First Look Survey. The maps are correlated
with 100 micron images measured with IRAS. At 24 and 70 micron, the spatial
power spectrum of surface brightness follows a power law with spectral index
-3.5. At 24 micron, the power law behavior is remarkably consistent from the
0.2 degree size of our maps down to the 5 arcsecond spatial resolution. Thus,
the structure of the 24 micron emission is self-similar even at milliparsec
scales. The combined power spectrum produced from Spitzer 24 micron and IRAS 25
micron images is consistent with a change in the power law exponent from -2.6
to -3.5. The decrease may be due to the transition from a two-dimensional to
three-dimensional structure. Under this hypothesis, we estimate the thickness
of the emitting medium to be 0.3 pc.Comment: 13 Pages, 3 Figures, to be published in Astrophysical Journal
Supplement Series (Spitzer Special Issue), volume 154. Uses aastex v5.
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