1,263 research outputs found
An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-Through-UV Curve Morphology
We study the IR-through-UV interstellar extinction curves towards 328
Galactic B and late-O stars. We use a new technique which employs stellar
atmosphere models in lieu of unreddened "standard" stars. This technique is
capable of virtually eliminating spectral mismatch errors in the curves. It
also allows a quantitative assessment of the errors and enables a rigorous
testing of the significance of relationships between various curve parameters,
regardless of whether their uncertainties are correlated. Analysis of the
curves gives the following results: (1) In accord with our previous findings,
the central position of the 2175 A extinction bump is mildly variable, its
width is highly variable, and the two variations are unrelated. (2) Strong
correlations are found among some extinction properties within the UV region,
and within the IR region. (3) With the exception of a few curves with extreme
(i.e., large) values of R(V), the UV and IR portions of Galactic extinction
curves are not correlated with each other. (4) The large sightline-to-sightline
variation seen in our sample implies that any average Galactic extinction curve
will always reflect the biases of its parent sample. (5) The use of an average
curve to deredden a spectral energy distribution (SED) will result in
significant errors, and a realistic error budget for the dereddened SED must
include the observed variance of Galactic curves. While the observed large
sightline-to-sightline variations, and the lack of correlation among the
various features of the curves, make it difficult to meaningfully characterize
average extinction properties, they demonstrate that extinction curves respond
sensitively to local conditions. Thus, each curve contains potentially unique
information about the grains along its sightline.Comment: To appear in the Astrophysical Journal, Part 1, July 1, 2007. Figures
and Tables which will appear only in the electronic version of the Journal
can be obtained via anonymous ftp from ftp://ftp.astronomy.villanova.edu .
After logging in, change directories to "fitz/FMV_EXTINCTION". A README file
describes the various files present in the director
A Search for Wolf-Rayet Stars in the Small Magellanic Cloud
We conducted an extensive search for Wolf-Rayet stars (W-Rs) in the SMC,
using the same interference filter imaging techniques that have proved
successful in finding W-Rs in more distant members of the Local Group.
Photometry of some 1.6 million stellar images resulted in some 20 good
candidates, which we then examined spectroscopically. Two of these indeed
proved to be newly found W-Rs, bringing the total known in the SMC from 9 to
11. Other finds included previously unknown Of-type stars (one as early as
O5f?p)),the recovery of the Luminous Blue Variable S18, and the discovery of a
previously unknown SMC symbiotic star. More important, however, is the fact
that there does not exist a significant number of W-Rs waiting to be discovered
in the SMC. The number of W-Rs in the SMC is a factor of 3 lower than in the
LMC (per unit luminosity), and we argue this is the result of the SMC's low
metallicity on the evolution of the most massive stars.Comment: Accepted by Astrophysical Journal. Postscript version available via
ftp.lowell.edu/pub/massey/smcwr.ps.gz Revised version contains slightly
revised spectral types for the Of stars but is otherwise unchange
Supersoft X-ray Sources in M31: I. A Chandra Survey and an Extension to Quasisoft Sources
We report on very soft X-ray sources (VSSs) in M31. In a survey which was
most sensitive to soft sources in four 8'x8' regions covered by Chandra's
ACIS-S S3 CCD, we find 33 VSSs that appear to belong to M31. Fifteen VSSs have
spectral characteristics mirroring the supersoft X-ray sources studied in the
Magellanic Cloud and Milky Way ( eV); we therefore call
these ``classical'' supersoft sources, or simply supersoft sources (SSSs).
Eighteen VSSs may either have small (< 10%) hard components, or slightly higher
effective temperatures (but still < 350 eV). We refer to these VSSs as
quasisoft sources (QSSs). While hot white dwarf models may apply to SSSs, the
effective temperatures of QSSs are too high, unless, e.g., the radiation
emanates from only a small portion of surface. Two of the SSSs were first
detected and identified as such through ROSAT observations. One SSS and one QSS
may be identified with symbiotics, and 2 SSSs with supernova remnants. Both
SSSs and QSSs in the disk are found near star-forming regions, possibly
indicating that they are young. VSSs in the outer disk and halo are likely to
be old systems; in these regions, there are more QSSs than SSSs, which is
opposite to what is found in fields closer to the galaxy center. The largest
density of bright VSSs is in the bulge; some of the bulge sources are close
enough to the nucleus to be remnants of the tidal disruption of a giant by the
massive central black hole. By using Chandra data in combination with ROSAT and
XMM observations, we find most VSSs to be highly variable, fading from or
brightening toward detectability on time scales of months. There is evidence
for VSSs with low luminosities ( erg s).Comment: 14 pages, 4 figures. Accepted for publication in ApJ. Classification
of supersoft and quasisoft sources is clarifie
Infrared Emission from Interstellar Dust. II. The Diffuse Interstellar Medium
We present a quantitative model for the infrared emission from dust in the
diffuse interstellar medium. The model consists of a mixture of amorphous
silicate grains and carbonaceous grains, each with a wide size distribution
ranging from molecules containing tens of atoms to large grains > 1 um in
diameter. We assume that the carbonaceous grains have polycyclic aromatic
hydrocarbon (PAH)-like properties at very small sizes, and graphitic properties
for radii a > 50 A. On the basis of recent laboratory studies and guided by
astronomical observations, we propose "astronomical" absorption cross sections
for use in modeling neutral and ionized PAHs from the far ultraviolet to the
far infrared. We also propose modifications to the far-infrared emissivity of
"astronomical silicate". We calculate energy distribution functions for small
grains undergoing "temperature spikes" due to stochastic absorption of
starlight photons, using realistic heat capacities and optical properties.
Using a grain size distribution consistent with the observed interstellar
extinction, we are able to reproduce the near-IR to submillimeter emission
spectrum of the diffuse interstellar medium, including the PAH emission
features at 3.3, 6.2, 7.7, 8.6, and 11.3um. The model is compared with the
observed emission at high Galactic latitudes as well as in the Galactic plane,
as measured by COBE and IRTS. We calculate infrared emission spectra for our
dust model heated by a range of starlight intensities, and we provide tabulated
dust opacities (extended tables available at
http://www.astro.princeton.edu/~draine/dust/dustmix.html)Comment: Final version published in ApJ, 554, 778 but with factor 1.086 error
in Table 6 and Fig. 16 corrected. Main change from astro-ph version 1 is
correction of typographical errors in Table 1, and correction of typo in eq.
(A2). 51 pages, 16 figures, Late
SPITZER SAGE Observations of Large Magellanic Cloud Planetary Nebulae
We present IRAC and MIPS images and photometry of a sample of previously
known planetary nebulae (PNe) from the SAGE survey of the Large Magellanic
Cloud (LMC) performed with the Spitzer Space Telescope. Of the 233 known PNe in
the survey field, 185 objects were detected in at least two of the IRAC bands,
and 161 detected in the MIPS 24 micron images. Color-color and color-magnitude
diagrams are presented using several combinations of IRAC, MIPS, and 2MASS
magnitudes. The location of an individual PN in the color-color diagrams is
seen to depend on the relative contributions of the spectral components which
include molecular hydrogen, polycyclic aromatic hydrocarbons (PAHs), infrared
forbidden line emission from the ionized gas, warm dust continuum, and emission
directly from the central star. The sample of LMC PNe is compared to a number
of Galactic PNe and found to not significantly differ in their position in
color-color space. We also explore the potential value of IR PNe luminosity
functions (LFs) in the LMC. IRAC LFs appear to follow the same functional form
as the well-established [O III] LFs although there are several PNe with
observed IR magnitudes brighter than the cut-offs in these LFs.Comment: 18 pages, 10 figures, 3 tables, to be published in the Astronomical
Journal. Additional online data available at
http://www.cfa.harvard.edu/irac/publications
A Reanalysis of theUltraviolet Extinction from Interstellar Dust in the Large Magellanic Cloud
We have reanalyzed the Large Magellanic Cloud's (LMC) ultraviolet (UV)
extinction using data from the IUE final archive. Our new analysis takes
advantage of the improved signal--to--noise of the IUE NEWSIPS reduction, the
exclusion of stars with very low reddening, the careful selection of well
matched comparison stars, and an analysis of the effects of Galactic foreground
dust. Differences between the average extinction curves of the 30 Dor region
and the rest of the LMC are reduced compared to previous studies. We find that
there is a group of stars with very weak 2175 Ang. bumps that lie in or near
the region occupied by the supergiant shell, LMC 2, on the southeast side of 30
Dor. The average extinction curves inside and outside LMC 2 show a very
significant difference in 2175 Ang. bump strength, but their far--UV
extinctions are similar. While it is unclear whether or not the extinction
outside the LMC 2 region can be fit with the relation of Cardelli, Clayton and
Mathis (CCM), sightlines near LMC 2 cannot be fit with CCM due to their weak
2175 Ang. bumps. While the extinction properties seen in the LMC lie within the
range of properties seen in the Galaxy, the correlations of UV extinction
properties with environment seen in the Galaxy do not appear to hold in the
LMC.Comment: 29 pages, 10 figures, to be published in Ap
Space Telescope Imaging Spectrograph Parallel Observations of the Planetary Nebula M94-20
The planetary nebula M94--20 in the Large Magellanic Cloud was
serendipitously observed with the Space Telescope Imaging Spectrograph on board
the Hubble Space Telescope as part of the Hubble Space Telescope Archival Pure
Parallel Program. We present spatially resolved imaging and spectral data of
the nebula and compare them with ground based data, including detection of
several emission lines from the nebula and the detection of the central star.
We find the total H alpha + [NII] flux = 7.3e-15 erg s^-1 cm^-2 and we estimate
the magnitude of the central star to be m_V = 26.0 +/- 0.2. Many other H alpha
sources have been found in M31, M33 and NGC 205 as well. We discuss the use of
the parallel observations as a versatile tool for planetary nebula surveys and
for other fields of astronomical research.Comment: Latex, 14 pages, 2 JPEG figures, 2 tables. PASP Research Note, June
1999, in pres
Dust Grain-Size Distributions From MRN to MEM
Employing the Maximum Entropy Method algorithm, we fit interstellar
extinction measurements which span the wavelength range 0.125-3 micron. We
present a uniform set of MEM model fits, all using the same grain materials,
optical constants and abundance constraints. In addition, we are taking
advantage of improved UV and IR data and better estimates of the gas-to-dust
ratio. The model fits cover the entire range of extinction properties that have
been seen in the Galaxy and the Magellanic Clouds. The grain models employed
for this presentation are the simplistic homogeneous spheres models (i.e.,
Mathis, Rumpl, & Nordsieck 1977) with two (graphite, silicate) or three
(graphite, silicate, amorphous carbon) components. Though such usage is only a
first step, the results do provide interesting insight into the use of grain
size as a diagnostic of dust environment. We find that the SMC Bar extinction
curve cannot be fit using carbon grains alone. This is a challenge to the
recent observational result indicating little silicon depletion in the SMC.Comment: 24 pages, 5 figures, accepted for publication in the Astrophysical
Journa
Correcting for the Effects of Interstellar Extinction
This paper addresses the issue of how best to correct astronomical data for
the wavelength-dependent effects of Galactic interstellar extinction. The main
general features of extinction from the IR through the UV are reviewed, along
with the nature of observed spatial variations. The enormous range of
extinction properties found in the Galaxy, particularly in the UV spectral
region, is illustrated. Fortunately, there are some tight constraints on the
wavelength dependence of extinction and some general correlations between
extinction curve shape and interstellar environment. These relationships
provide some guidance for correcting data for the effects of extinction.
Several strategies for dereddening are discussed along with estimates of the
uncertainties inherent in each method. In the Appendix, a new derivation of the
wavelength dependence of an average Galactic extinction curve from the IR
through the UV is presented, along with a new estimate of how this extinction
law varies with the parameter R = A(V)/E(B-V). These curves represent the true
monochromatic wavelength dependence of extinction and, as such, are suitable
for dereddening IR--UV spectrophotometric data of any resolution, and can be
used to derive extinction relations for any photometry system.Comment: To appear in PASP (January 1999) 14 pages including 4 pages of
figures Uses emulateapj style. PASP, in press (January 1999
New Interstellar Dust Models Consistent with Extinction, Emission, and Abundance Constraints
We present new interstellar dust models which have been derived by
simultaneously fitting the far-ultraviolet to near-infrared extinction, the
diffuse infrared (IR) emission and, unlike previous models, the elemental
abundance constraints on the dust for different interstellar medium abundances,
including solar, F and G star, and B star abundances. The fitting problem is a
typical ill-posed inversion problem, in which the grain size distribution is
the unknown, which we solve by using the method of regularization. The dust
model contains various components: PAHs, bare silicate, graphite, and amorphous
carbon particles, as well as composite particles containing silicate, organic
refractory material, water ice, and voids. The optical properties of these
components were calculated using physical optical constants. As a special case,
we reproduce the Li & Draine (2001) results, however their model requires an
excessive amount of silicon, magnesium, and iron to be locked up in dust: about
50 ppm (atoms per million of H atoms), significantly more than the upper limit
imposed by solar abundances of these elements, about 34, 35, and 28 ppm,
respectively. A major conclusion of this paper is that there is no unique
interstellar dust model that simultaneously fits the observed extinction,
diffuse IR emission, and abundances constraints.Comment: 70 pages, 23 figures, accepted for publication in the Astrophysical
Journal Supplemen
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