We present Hubble Space Telescope Space Telescope Imaging Spectrograph far-UV spectra of the edge-on disk around 49 Ceti, one of the very few debris disks showing submillimeter CO emission. Many ...atomic absorption lines are present in the spectra, most of which arise from circumstellar gas lying along the line-of-sight to the central star. We determined the line-of-sight C I column density, estimated the total carbon column density, and set limits on the O I column density. Surprisingly, no line-of-sight CO absorption was seen. We discuss possible explanations for this non-detection, and present preliminary estimates of the carbon abundances in the line-of-sight gas. The C/Fe ratio is much greater than the solar value, suggesting that 49 Cet harbors a volatile-rich gas disk similar to that of beta Pictoris.
CaII K-line (3933 Å) absorption profiles observed towards 15 A-type and two B-type stars with known (or suspected) debris disks, in addition to archival spectral data for three A-type stars, are ...presented. Inspection of the extracted CaII K-line absorption lines has determined that one late B-type (HD 58647) and four new A-type (HD 56537, HD 64145, HD 108767, and HD 109573) stellar systems exhibit short-term (night-to-night) absorption variation within these profiles. This variability is due to the liberation of gas from falling evaporating bodies (the so-called FEBs activity) on the grazing approach of exocomets towards their parent star. Also we present archival spectra of the FeI λ3860 Å and CaII K-lines towards β Pictoris (HD 39060) that, for the first time, reveal the absorption signatures of FEB activity at similar velocities in both line profiles. FEB-hosting stars seem to be of an earlier type (on average spectral type Al.8 as compared with A3.1) than those where no FEB activity has been observed. This could be due to stellar activity levels (such as chromospheric activity or nonradial pulsations) being of a higher level within the atmospheres of these hotter A-type stars.
We have initiated a search for extended ultraviolet disk (XUV-disk) galaxies in the local universe. Here we compare GALEX UV and visible-NIR images of 189 nearby (D < 40 Mpc) S0-Sm galaxies Included ...in the GALEX Atlas of Nearby Galaxies and present the first catalog of XUV-disk galaxies. We find that XUV-disk galaxies are surprisingly common but have varied relative (UV/optical) extent and morphology. Type 1 objects ( unk 20% incidence) have structured, UV-bright/optically faint emission features in the outer disk, beyond the traditional star formation threshold. Type 2 XUV-disk galaxies ( similar to 10% incidence) exhibit an exceptionally large, UV-bright/optically low surface brightness (LSB) zone having blue UV-K sub(S) outside the effective extent of the inner, older stellar population, but not reaching extreme galactocentric distance. If the activity occurring in XUV-disks is episodic, a higher fraction of present-day spirals could be influenced by such outer disk star formation. Type 1 disks are associated with spirals of all types, whereas Type 2 XUV-disks are predominantly found In late-type spirals. Type 2 XUV-disks are forming stars quickly enough to double their (currently low) stellar mass in the next Gyr (assuming a constant star formation rate). XUV-disk galaxies of both types are systematically more gas-rich than the general galaxy population. Minor external perturbation may stimulate XUV-disk incidence, at least for Type 1 objects. XUV-disks are the most actively evolving galaxies growing via inside-out disk formation in the current epoch, and may constitute a segment of the galaxy population experiencing significant, continued gas accretion from the intergalactic medium or neighboring objects.
Interstellar Gas in the Local Chimneys Welsh, Barry Y.; Lallement, Rosine
Publications of the Astronomical Society of the Pacific,
06/2012, Letnik:
124, Številka:
916
Journal Article
Recenzirano
ABSTRACT We present ultraviolet absorption observations of four nearby (d < 250 pc) DA white dwarfs using spectral data (R ∼ 15 km s-1) obtained with the NASA HST Cosmic Origins Spectrograph ...(1150-1700 Å) and the Far Ultraviolet Spectroscopic Explorer (912-1170 Å). Two of these targets (WD2321 - 549 and WD2124 - 224) sample gas along the Southern Local Chimney sight line, while WD1040 + 492 samples gas within the Northern Local Chimney and the sight line toward WD2257 - 073 samples gas located entirely within the central confines of the Local Interstellar Cavity. We have fit the numerous high and low ion interstellar absorption-line profiles with theoretical models in order to determine the velocity and ion column densities of the sight-line gas clouds. Using these data we have derived element abundances relative to that of (assumed undepleted) sulfur for each of the four sight lines. We find that the 89 pc sight line toward WD2257 - 73 reveals a pattern of element depletion associated with interstellar gas located within the inner Local Cavity that is greater than those derived for the other three sight lines that sample gas within the two Local Chimney features. The sight lines toward WD2124 - 224 and WD2321 - 549, which sample gas along the Southern Local Chimney, show patterns of element depletion that are greater than that derived for the Northern Local Chimney sight line toward WD1040 + 492. Although our sight line sample is small, these data seem to indicate that the interstellar gas located along both local chimneys may be chemically different from gas within the central regions of the Local Cavity. This may indicate that Local Chimney gas has a different physical history and origin from the interstellar gas located within the inner Local Cavity.
We present medium resolution ultraviolet absorption measurements recorded with the HST-COS and FUSE spectrographs towards the Wolf-Rayet star UIT-236 belonging to the BCLMP 99 HII region in the M33 ...galaxy. We have detected at least 5 major absorption systems along the 840 kpc sight-line to UIT-236 with average gas cloud LSR velocities of -205, -170, -135, -32, and +3 km s-1. The three most negative velocity cloud components, which we associate with gas in the M33 galaxy, have been detected in the line profiles of CII, CII∗,CIII, NI, NII, OI, OVI, SiII, SiII∗, SiIII, SiIV, PII, SII, ArI, FeII, and FeIII ions. These profiles have been fit with absorption models to determine ion column densities for each cloud component. For the case of highly ionized gas components, the observed column density ratios of OVI/SiIV are most compatible with the predictions of ionization by turbulent mixing layers or in a shock driven medium. A possible formation scenario is that the SiIV is formed within a photo-ionized stellar wind bubble that is being driven into surrounding gas that collisionally produces the observed OVI ions. Using the column density ratios of the CII/CII∗ lines, we have determined electron density values in the range 1.8-6.0 cm-3 for the ionized M33 gas clouds. Such values are similar to those found for other ionized interstellar bubbles such as N51D (in the Large Magellanic Cloud), γ2 Vel, and NGC 2244 (the Rosette Nebula). We have carried out a gas phase element abundance analysis of the three cloud components, indicating that C, N, O, Si, P, Ar, Fe, and Ni are all depleted relative to their solar abundance values. This pattern is confirmed by the observed N(OI)/N(HI) column density ratios which suggest a low metallicity for all three of the neutral gas components. The presence of interstellar shocks and expanding supershells of gas driven by stellar wind outflows from stars like UIT-236 may well account for the element depletion values observed along the M33 sight-line.
For a sample of 43 nearby, late-type galaxies, we have investigated the radial variation of both the current star formation rate and the dust-induced UV light attenuation. To do this we have ...cross-correlated IRAS images and GALEX observations for each of these galaxies and compiled observations of the gas (CO and H I) and metal-abundance gradients found in the literature. We find that attenuation correlates with metallicity. We then use the UV profiles, corrected for attenuation, to study several variants of the Schmidt law and conclude that our results are compatible with a simple law similar to the one of Kennicutt extending smoothly to lower surface densities, but with considerable scatter. We do not detect an abrupt break in the UV light at the threshold radius derived from H alpha data (at which the H alpha profile shows a break and beyond which only a few H II regions are usually found). We interpret the H alpha sudden break not as a change in the star formation regime (as often suggested), but as the vanishingly small number of ionizing stars corresponding to low levels of star formation.
The trouble with the Local Bubble Welsh, Barry Y.; Shelton, Robin L.
Astrophysics and space science,
09/2009, Letnik:
323, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The model of a Local Hot Bubble has been widely accepted as providing a framework that can explain the ubiquitous presence of the soft X-ray background diffuse emission. We summarize the current ...knowledge on this local interstellar region, paying particular reference to observations that sample emission from the presumed local million degree K hot plasma. However, we have listed numerous observations that are seemingly in conflict with the concept of a hot Local Bubble. In particular, the discovery of solar wind charge exchange that can generate an appreciable soft X-ray background signal within the heliosphere, has led to a re-assessment of the generally accepted model that requires a hot local plasma.
In order to explain the majority of observations of the local plasma, we forward two new speculative models that describe the physical state of the local interstellar gas. One possible scenario is similar to the present widely accepted model of the Local Hot Bubble, except that it accounts for only 50% of the soft X-ray emission currently detected in the galactic plane, has a lower thermal pressure than previously thought, and its hot plasma is not as hot as previously believed. Although such a model can solve several difficulties with the traditional hot Local Bubble model, a heating mechanism for the dimmer and cooler gas remains to be found. The second possible explanation is that of the ‘Hot Top’ model, in which the Local Cavity is an old supernova remnant in which no (or very little) million degree local plasma is presently required. Instead, the cavity is now thought to be filled with partially ionized cloudlets of temperature ∼7000 K that are surrounded by lower density envelopes of photo-ionized gas of temperature ∼20,000 K. Although this new scenario provides a natural explanation for many of the observations that were in conflict with the Local Hot Bubble model, we cannot (as yet) provide a satisfactory explanation or the emission levels observed in the B and Be ultra-soft X-ray bands.
Ultraviolet imaging with the Galaxy Evolution Explorer (GALEX) has revealed an extensive sample of UV-bright stellar complexes in the extreme outer disk of M83, extending to about four times the ...radius where the majority of HII regions are detected (R_HII = 5.1\' or 6.6 kpc). These sources are typically associated with large-scale filamentary HI structures in the warped outer disk of M83, and are distributed beyond the galactocentric radii at which molecular ISM has yet been detected. We present measured properties of these stellar complexes, including FUV and NUV magnitudes and local gas surface density. Only a subset of the outer disk UV sources have corresponding HII regions detected in H-alpha imaging, consistent with a sample of mixed age in which some sources are a few Myr old and others are much more evolved (~ 10^8 yr).
We have used the first matched set of GALEX and SDSS data to investigate the properties of a sample of 74 nearby galaxies with far-ultraviolet luminosities chosen to overlap the luminosity range of ...typical high-z Lyman Break Galaxies (LBGs). GALEX deep surveys have shown that ultraviolet-luminous galaxies (UVLGs) similar to these are the fastest evolving component of the UV galaxy population. Model fits to the combined GALEX and SDSS photometry yield typical FUV extinctions similar to LBGs. The implied star formation rates are SFR ~ 3 to 30 solar mass per year. This overlaps the range of SFRs for LBGs. We find a strong inverse correlation between galaxy mass and far-ultraviolet surface brightness, and on this basis divide the sample into ``large\'\' and ``compact\'\' UVLGs. The compact UVLGs have half-light radii of a few kpc or less (similar to LBGs). They are relatively low mass galaxies (~10 billion solar masses) with typical velocity dispersions of 60 to 150 km/s. They span a range in metallicity from 0.3 to 1 times solar, have blue optical-UV colors, and are forming stars at a rate sufficient to build the present galaxy in ~a Gigayear. In all these respects they appear similar to the LBG population. These ``living fossils\'\' may therefore provide an opportunity for detailed investigation of the physical processes occurring in typical star forming galaxies in the early universe.
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