94 research outputs found
Origin of the Galactic Disk 6.7 kev Line Emission
The goal of this program was to determine if the extended FeXXV 6.7 kev line emission might possibly be produced and confined by the hot wind-shocked bubbles to accompany UC HII regions. The main result of this study are: (1) FeXXV is detected in the W3 complex, but at a level that could only explain a small fraction of the galactic disk emission if all UC HII regions emit at about the same intensity as the W3 complex; (2) Two X-ray sources are detected in W3. W3-X 1 coincides with the radio image of this region, but W3-X2 has no radio, optical, or infrared counterpart; (3) There is no evidence for variability of W3-X1 during the period of observations (approx, 40,000 sec); (4) The X-ray spectrum of W3-X1 has no emission shortward of 1 kev, it peaks at approx. 2 kev and show significant emission out to approx. 6 kev. No individual lines are resolved. There is currently no generally accepted theory for extended hard X-ray emission in HII regions. Perhaps the most significant discovery of this program has been the detection of extended hard X-rays and the realization that some entirely new processes must be invoked to understand this; and (5)A minimum (chi)(sup 2) fit of the spectrum implies a H absorbing column of N(sub H) approx, equals to 2.1 x 10(exp 22)/ cm, a temperature of the emitting plasma of 7 x 10(exp 7) K, and a luminosity of approx. equal to 10(33)erg/s
Infrared emission from ultracompact H II regions
Models of circumstellar dust shells around ultracompact (UC) H II regions were constructed that accurately fit the observed IR flux distributions. The models assume spherically symmetric dust shells illuminated by stars whose bolometric luminosity is inferred from the integrated FIR flux densities. Assuming ionization by a single zero age main sequence (ZAMS) star, the relations of Panagia were used to infer the stellar radius and effective temperature for a given luminosity. The grain mixture in the dust shell consists of bare graphite and silicate grains with the optical properties of Draine and Lee and the size distribution of Mathis et al. The computer code of Wolfire et al was used to solve the radiative transfer equations through a spherical dust shell. The model provides monochromatic luminosities, dust temperatures, and opacities through the shell. Aside from the stellar and dust properties, the only other input parameters to the model are the distance to the shell, the form of its density distribution, and its outer radius. Predictions of the model are compared with observations of a typical UC H II region and the run of dust temperature with radius and the optical depth with frequency are discussed
Infrared point source variability between the Spitzer and MSX surveys of the Galactic mid-plane
We present a list of 552 sources with suspected variability, based on a
comparison of mid-infrared photometry from the GLIMPSE I and MSX surveys, which
were carried out nearly a decade apart. We were careful to address issues such
as the difference in resolution and sensitivity between the two surveys, as
well as the differences in the spectral responses of the instruments. We
selected only sources where the IRAC 8.0 and MSX 8.28 micron fluxes differ by
more than a factor of two, in order to minimize contamination from sources
where the difference in fluxes at 8 micron is due to a strong 10 micron
silicate feature. We present a subset of 40 sources for which additional
evidence suggests variability, using 2MASS and MIPSGAL data. Based on a
comparison with the variability flags in the IRAS and MSX Point-Source Catalogs
we estimate that at least a quarter of the 552 sources, and at least half of
the 40 sources are truly variable. In addition, we tentatively confirm the
variability of one source using multi-epoch IRAS LRS spectra. We suggest that
most of the sources in our list are likely to be Asymptotic Giant Branch stars.Comment: 47 pages, 12 Figures, 3 Tables, accepted for publication in A
A Massive Protostar Forming by Ordered Collapse of a Dense, Massive Core
We present 30 and 40 micron imaging of the massive protostar G35.20-0.74 with
SOFIA-FORCAST. The high surface density of the natal core around the protostar
leads to high extinction, even at these relatively long wavelengths, causing
the observed flux to be dominated by that emerging from the near-facing outflow
cavity. However, emission from the far-facing cavity is still clearly detected.
We combine these results with fluxes from the near-infrared to mm to construct
a spectral energy distribution (SED). For isotropic emission the bolometric
luminosity would be 3.3x10^4 Lsun. We perform radiative transfer modeling of a
protostar forming by ordered, symmetric collapse from a massive core bounded by
a clump with high mass surface density, Sigma_cl. To fit the SED requires
protostellar masses ~20-34 Msun depending on the outflow cavity opening angle
(35 - 50 degrees), and Sigma_cl ~ 0.4-1 g cm-2. After accounting for the
foreground extinction and the flashlight effect, the true bolometric luminosity
is ~ (0.7-2.2)x10^5 Lsun. One of these models also has excellent agreement with
the observed intensity profiles along the outflow axis at 10, 18, 31 and 37
microns. Overall our results support a model of massive star formation
involving the relatively ordered, symmetric collapse of a massive, dense core
and the launching bipolar outflows that clear low density cavities. Thus a
unified model may apply for the formation of both low and high mass stars.Comment: 6 pages, 4 figures, 1 table, accepted to Ap
First Detection of an H2CO 6 cm Maser Flare: A Burst in IRAS 18566+0408
We report the discovery of a short-duration (less than 3 months) outburst of the H2CO 6 cm maser in IRAS 18566+0408 (G37.55+0.20). During the flare, the peak flux density of the maser increased by a factor of 4; after less than a month, it decayed to the preflare value. This is the first detection of a short, burstlike variability of an H2CO 6 cm maser. The maser shows an asymmetric line profile that is consistent with the superposition of two Gaussian components. We did not detect a change in the velocity or the line width of the Gaussian components during the flare. If the two Gaussian components trace two separate maser regions, then very likely an event outside the maser gas triggered simultaneous flares at two different locations
Free-Free Spectral Energy Distributions of Hierarchically Clumped HII Regions
In an effort to understand unusual power-law spectral slopes observed in some
hypercompact HII regions, we consider the radio continuum energy distribution
from an ensemble of spherical clumps. An analytic expression for the free-free
emission from a single spherical clump is derived. The radio continuum slope
(with F_\nu \nu^\alpha) is governed by the population of clump optical depths
N(tau), such that (a) at frequencies where all clumps are thick, a continuum
slope of +2 is found, (b) at frequencies where all clumps are optically thin, a
flattened slope of -0.11 is found, and (c) at intermediate frequencies, a
power-law segment of significant bandwidth with slopes between these two
limiting values can result. For the ensemble distribution, we adopt a power-law
distribution N(tau) tau^{-\gamma}, and find that significant power-law segments
in the SED with slopes from +2 to -0.11 result only for a relatively restricted
range of values of 1 to 2. Further, a greater range of clump optical
depths for this distribution leads to a wider bandwidth over which the
intermediate power-law segment exists. The model is applied to the source
W49N-B2 with an observed slope of \alphab +0.9, but that may be turning over to
become optically thin around 2 mm. An adequate fit is found in which most
clumps are optically thin and there is little shadowing of rearward clumps by
foreground clumps (i.e., the geometrical covering factor C<<1). The primary
insight gained from our study is that in the Rayleigh-Jeans limit for the
Planck function that applies for the radio band, it is the distribution in
optical depth of the clump population that is solely responsible for setting
the continuum shape, with variations in the size and temperature of clumps
serving to modulate the level of free-free emission.Comment: Astrophysical Journal, in pres
Spitzer Space Telescope observations of the Carina Nebula: The steady march of feedback-driven star formation
We report the first results of imaging the Carina Nebula with Spitzer/IRAC,
providing a catalog of point sources and YSOs based on SED fits. We discuss
several aspects of the extended emission, including dust pillars that result
when a clumpy molecular cloud is shredded by massive star feedback. There are
few "extended green objects" (EGOs) normally taken as signposts of outflow
activity, and none of the HH jets detected optically are seen as EGOs. A
population of "extended red objects" tends to be found around OB stars, some
with clear bow-shocks. These are dusty shocks where stellar winds collide with
flows off nearby clouds. Finally, the relative distributions of O stars and
subclusters of YSOs as compared to dust pillars shows that while some YSOs are
located within pillars, many more stars and YSOs reside just outside pillar
heads. We suggest that pillars are transient phenomena, part of a continuous
outwardly propagating wave of star formation driven by massive star feedback.
As pillars are destroyed, they leave newly formed stars in their wake, which
are then subsumed into the young OB association. Altogether, the current
generation of YSOs shows no strong deviation from a normal IMF. The number of
YSOs suggests a roughly constant star-formation rate over the past 3Myr,
implying that star formation in pillars constitutes an important mechanism to
construct unbound OB associations. Accelerated pillars may give birth to O-type
stars that, after several Myr, could appear to have formed in isolation.Comment: 25 pages, 15 figures, MNRAS accepte
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