137 research outputs found
FUSE Observations of the Dwarf Nova SW UMa During Quiescence
We present spectroscopic observations of the short-period cataclysmic
variable SW Ursa Majoris, obtained by the Far Ultraviolet Spectroscopic
Explorer (FUSE) satellite while the system was in quiescence. The data include
the resonance lines of O VI at 1031.91 and 1037.61 A. These lines are present
in emission, and they exhibit both narrow (~ 150 km/s) and broad (~ 2000 km/s)
components. The narrow O VI emission lines exhibit unusual double-peaked and
redshifted profiles. We attribute the source of this emission to a cooling flow
onto the surface of the white dwarf primary. The broad O VI emission most
likely originates in a thin, photoionized surface layer on the accretion disk.
We searched for emission from H_2 at 1050 and 1100 A, motivated by the
expectation that the bulk of the quiescent accretion disk is in the form of
cool, molecular gas. If H_2 is present, then our limits on the fluxes of the
H_2 lines are consistent with the presence of a surface layer of atomic H that
shields the interior of the disk. These results may indicate that accretion
operates primarily in the surface layers of the disk in SW UMa. We also
investigate the far-UV continuum of SW UMa and place an upper limit of 15,000 K
on the effective temperature of the white dwarf.Comment: 21 Pages, 3 figures, to be published in Ap
IR Dust Bubbles: Probing the Detailed Structure and Young Massive Stellar Populations of Galactic HII Regions
We present an analysis of wind-blown, parsec-sized, mid-infrared bubbles and
associated star-formation using GLIMPSE/IRAC, MIPSGAL/MIPS and MAGPIS/VLA
surveys. Three bubbles from the Churchwell et al. (2006) catalog were selected.
The relative distribution of the ionized gas (based on 20 cm emission), PAH
emission (based on 8 um, 5.8 um and lack of 4.5 um emission) and hot dust (24
um emission) are compared. At the center of each bubble there is a region
containing ionized gas and hot dust, surrounded by PAHs. We identify the likely
source(s) of the stellar wind and ionizing flux producing each bubble based
upon SED fitting to numerical hot stellar photosphere models. Candidate YSOs
are also identified using SED fitting, including several sites of possible
triggered star formation.Comment: 37 pages, 17 figure
The Bubbling Galactic Disk
A visual examination of the images from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) has revealed 322 partial and closed rings that we propose represent partially or fully enclosed three-dimensional bubbles. We argue that the bubbles are primarily formed by hot young stars in massive star formation regions. We have found an average of about 1.5 bubbles per square degree. About 25% of the bubbles coincide with known radio H II regions, and about 13% enclose known star clusters. It appears that B4-B9 stars (too cool to produce detectable radio H II regions) probably produce about three-quarters of the bubbles in our sample, and the remainder are produced by young O-B3 stars that produce detectable radio H II regions. Some of the bubbles may be the outer edges of H II regions where PAH spectral features are excited and may not be dynamically formed by stellar winds. Only three of the bubbles are identified as known SNRs. No bubbles coincide with known planetary nebulae or W-R stars in the GLIMPSE survey area. The bubbles are small. The distribution of angular diameters peaks between 1' and 3' with over 98% having angular diameters less than 10' and 88% less than 4'. Almost 90% have shell thicknesses between 0.2 and 0.4 of their outer radii. Bubble shell thickness increases approximately linearly with shell radius. The eccentricities are rather large, peaking between 0.6 and 0.7; about 65% have eccentricities between 0.55 and 0.85
The Frequency of Mid-Infrared Excess Sources in Galactic Surveys
We have identified 230 Tycho-2 Spectral Catalog stars that exhibit 8 micron
mid-infrared extraphotospheric excesses in the MidCourse Space Experiment (MSX)
and Spitzer Space Telescope Galactic Legacy MidPlane Survey Extraordinaire
(GLIMPSE) surveys. Of these, 183 are either OB stars earlier than B8 in which
the excess plausibly arises from a thermal bremsstrahlung component or evolved
stars in which the excess may be explained by an atmospheric dust component.
The remaining 47 stars have spectral classifications B8 or later and appear to
be main sequence or late pre-main-sequence objects harboring circumstellar
disks. Six of the 47 stars exhibit multiple signatures characteristic of
pre-main-sequence circumstellar disks, including emission lines, near-infrared
K-band excesses, and X-ray emission. Approximately one-third of the remaining
41 sources have emission lines suggesting relative youth. Of the 25 GLIMPSE
stars with SST data at >24 microns, 20 also show an excess at 24 microns. Three
additional objects have 24 micron upper limits consistent with possible
excesses, and two objects have photospheric measurements at 24 microns. Six MSX
sources had a measurement at wavelengths >8 microns. We modeled the excesses in
26 stars having two or more measurements in excess of the expected photospheres
as single-component blackbodies. We determine probable disk temperatures and
fractional infrared luminosities in the range 191 < T < 787 and 3.9x10^-4 <
L_IR/L_* < 2.7x10^-1. We estimate a lower limit on the fraction of Tycho-2
Spectral Catalog main-sequence stars having mid-IR, but not near-IR, excesses
to be 1.0+-0.3%.Comment: Accepted to Ap
The RMS Survey: Mid-Infrared Observations of Candidate Massive YSOs in the Southern Hemisphere
Abridged abstract: The Red MSX Source (RMS) survey is an ongoing effort to
return a large, well-selected sample of massive young stellar objects (MYSOs)
within our Galaxy. A series of ground-based follow-up observations are being
undertaken in order to remove contaminant objects from our list of 2000
candidates, and to begin characterising these MYSOs. As a part of these
follow-up observations, high resolution (~1") mid-IR imaging aids the
identification of contaminant objects which are resolved (UCHII regions, PN) as
opposed to those which are unresolved (YSOs, evolved stars) as well as
identifying YSOs near UCHII regions and other multiple sources. We present 10.4
micron imaging observations for 346 candidate MYSOs in the RMS survey in the
Southern Hemisphere, primarily outside the region covered by the GLIMPSE
Spitzer Legacy Survey. These were obtained using TIMMI2 on the ESO 3.6m
telescope in La Silla, Chile. Our photometric accuracy is of order 0.05Jy, and
our astrometric accuracy is 0.8", which is an improvement over the nominal 2"
accuracy of the MSX PSC.Comment: 9 page paper accepted to A&A. Online data for table 2 and figure 1
will be available in the published online version of this paper via A&A. The
paper contains 7 figures and 3 table
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