1,383 research outputs found
Star formation in the inner galaxy: A far-infrared and radio study of two H2 regions
Far-infrared and radio continuum maps have been made of the central 6' of the inner-galaxy H II regions G30.8-0.0 (in the W43 complex) and G25.4-02., along with radio and molecular line measurements at selected positions. An effort is made to understand far infrared wavelingths allow the dust temperature structures and total far infrared fluxes to be determined. Comparison of the radio and infrared maps shows a close relationship between the ionized gas and the infrared-emitting material. There is evidence that parts of G30.8 are substantially affected by extinction, even at far-infrared wavelengths. For G25.4-0.2, the radio recombination line and CO line data permit resolution of the distance ambiguity for this source. The confusion in distance determination is found to result from an extraordinary near-superposition of two bright H II regions. Using revised distances of 4.3 kpc for G26.4SE and 12 kpc for G25.4NW, that the latter, which is apparently the fainter of the two sources, is actually the more luminous. Though it is not seen on the Palomar Sky Survey, G25.4SE is easily visible in the 9532A line of S III and is mapped in this line. The ratio of total luminosity to ionizing luminosity is very similar to that of H II regions in the solar circle. Assuming a coeval population of ionizing stars, a normal initial mass function is indicated
Probing protoplanetary disks with silicate emission: Where is the silicate emission zone?
Recent results indicate that the grain size and crystallinity inferred from observations of silicate features may be correlated with the spectral type of the central star and/or disk geometry. In this paper, we show that grain size, as probed by the 10 μm silicate feature peak-to-continuum and 11.3 to 9.8 μm flux ratios, is inversely proportional to log Lsstarf. These trends can be understood using a simple two-layer disk model for passive irradiated flaring disks, CGPLUS. We find that the radius, R10, of the 10 μm silicate emission zone in the disk goes as (L*/L☉)^0.56, with slight variations depending on disk geometry (flaring angle and inner disk radius). The observed correlations, combined with simulated emission spectra of olivine and pyroxene mixtures, imply a dependence of grain size on luminosity. Combined with the fact that R10 is smaller for less luminous stars, this implies that the apparent grain size of the emitting dust is larger for low-luminosity sources. In contrast, our models suggest that the crystallinity is only marginally affected, because for increasing luminosity, the zone for thermal annealing (assumed to be at T > 800 K) is enlarged by roughly the same factor as the silicate emission zone. The observed crystallinity is affected by disk geometry, however, with increased crystallinity in flat disks. The apparent crystallinity may also increase with grain growth due to a corresponding increase in contrast between crystalline and amorphous silicate emission bands
Gaia-DR2 confirms VLBA parallaxes in Ophiuchus, Serpens and Aquila
We present Gaia-DR2 astrometry of a sample of YSO candidates in Ophiuchus,
Serpens Main and Serpens South/W40 in the Aquila Rift, which had been mainly
identified by their infrared excess with Spitzer. We compare the Gaia-DR2
parallaxes against published and new parallaxes obtained from our Very Long
Baseline Array (VLBA) program GOBELINS. We obtain consistent results between
Gaia and the VLBA for the mean parallaxes in each of the regions analyzed here.
We see small offsets, when comparing mean values, of a few tens of
micro-arcseconds in the parallaxes, which are either introduced by the Gaia
zero-point error or due to a selection effect by Gaia toward the brightest,
less obscured stars. Gaia-DR2 data alone conclusively places Serpens Main and
Serpens South at the same distance, as we first inferred from VLBA data alone
in a previous publication. Thus, Serpens Main, Serpens South and W40 are all
part of the same complex of molecular clouds, located at a mean distance of
436+/-9 pc. In Ophiuchus, both Gaia and VLBA suggest a small parallax gradient
across the cloud, and the distance changes from 144.2+/-1.3 pc to 138.4+/-2.6
pc when going from L1689 to L1688.Comment: Accepted for publication in ApJ
Shockingly low water abundances in Herschel / PACS observations of low-mass protostars in Perseus
Protostars interact with their surroundings through jets and winds impacting
on the envelope and creating shocks, but the nature of these shocks is still
poorly understood. Our aim is to survey far-infrared molecular line emission
from a uniform and significant sample of deeply-embedded low-mass young stellar
objects in order to characterize shocks and the possible role of ultraviolet
radiation in the immediate protostellar environment. Herschel/PACS spectral
maps of 22 objects in the Perseus molecular cloud were obtained as part of the
`William Herschel Line Legacy' survey. Line emission from HO, CO,
and OH is tested against shock models from the literature.
Observed line ratios are remarkably similar and do not show variations with
source physical parameters. Observations show good agreement with the shock
models when line ratios of the same species are compared. Ratios of various
HO lines provide a particularly good diagnostic of pre-shock gas
densities, cm, in agreement with typical
densities obtained from observations of the post-shock gas. The corresponding
shock velocities, obtained from comparison with CO line ratios, are above 20
km\,s. However, the observations consistently show one-to-two orders of
magnitude lower HO-to-CO and HO-to-OH line ratios
than predicted by the existing shock models.
The overestimated model HO fluxes are most likely caused by an
overabundance of HO in the models since the excitation is
well-reproduced. Illumination of the shocked material by ultraviolet photons
produced either in the star-disk system or, more locally, in the shock, would
decrease the HO abundances and reconcile the models with
observations. Detections of hot HO and strong OH lines support
this scenario.Comment: 28 pages, 12 figures, accepted to Astronomy & Astrophysic
The Gould's Belt Distances Survey (GOBELINS). V. Distances and Kinematics of the Perseus molecular cloud
We derive the distance and structure of the Perseus molecular cloud by
combining trigonometric parallaxes from Very Long Baseline Array (VLBA)
observations, taken as part of the GOBELINS survey, and Gaia Data Release 2.
Based on our VLBA astrometry, we obtain a distance of 321+/-10 pc for IC 348.
This is fully consistent with the mean distance of 320+/-26 measured by Gaia.
The VLBA observations toward NGC 1333 are insufficient to claim a successful
distance measurement to this cluster. Gaia parallaxes, on the other hand, yield
a mean distance of 293+/-22 pc. Hence, the distance along the line of sight
between the eastern and western edges of the cloud is ~30 pc, which is
significantly smaller than previously inferred. We use Gaia proper motions and
published radial velocities to derive the spatial velocities of a selected
sample of stars. The average velocity vectors with respect to the LSR are
(u,v,w) = (-6.1+/-1.6, 6.8+/-1.1, -0.9+/-1.2) and (-6.4+/-1.0, 2.1+/-1.4,
-2.4+/-1.0) km/s for IC 348 and NGC 1333, respectively. Finally, our analysis
of the kinematics of the stars has shown that there is no clear evidence of
expansion, contraction, or rotational motions within the clusters.Comment: Accepted for publication in Ap
- …