82 research outputs found
Early phase of massive star formation: A case study of Infrared dark cloud G084.81-01.09
We mapped the MSX dark cloud G084.81-01.09 in the NH3 (1,1) - (4,4) lines and
in the J = 1-0 transitions of 12CO, 13CO, C18O and HCO+ in order to study the
physical properties of infrared dark clouds, and to better understand the
initial conditions for massive star formation. Six ammonia cores are identified
with masses ranging from 60 to 250 M_sun, a kinetic temperature of 12 K, and a
molecular hydrogen number density n(H2) ~ 10^5 cm^-3. In our high mass cores,
the ammonia line width of 1 km/s is larger than those found in lower mass cores
but narrower than the more evolved massive ones. We detected self-reversed
profiles in HCO+ across the northern part of our cloud and velocity gradients
in different molecules. These indicate an expanding motion in the outer layer
and more complex motions of the clumps more inside our cloud. We also discuss
the millimeter wave continuum from the dust. These properties indicate that our
cloud is a potential site of massive star formation but is still in a very
early evolutionary stage
Present and Future Gamma-Ray Probes of the Cygnus OB2 Environment
The MAGIC Collaboration has provided new observational data pertaining to the
TeV J2032+4130 gamma-ray source (within the Cygnus OB2 region), for energies
E_gamma >400 GeV. It is then appropriate to update the impact of these data on
gamma-ray production mechanisms in stellar associations. We consider two
mechanisms of gamma-ray emission, pion production and decay (PION) and
photo-excitation of high-energy nuclei followed by prompt photo-emission from
the daughter nuclei (A*). We find that while the data can be accommodated with
either scenario, the A* features a spectral bump, corresponding to the
threshold for exciting the Giant Dipole Resonance, which can serve to
discriminate between them. We comment on neutrino emission and detection from
the region if the PION and/or A* processes are operative. We also touch on the
implications for this analysis of future Fermi and Cerenkov Telescope Array
data.Comment: 6 pp, 2 figs. Matching version publihed in Phys. Rev.
The cooling of atomic and molecular gas in DR21
We present an overview of a high-mass star formation region through the major
(sub-)mm, and far-infrared cooling lines to gain insight into the physical
conditions and the energy budget of the molecular cloud. We used the KOSMA 3m
telescope to map the core () of the Galactic star forming region
DR 21/DR 21 (OH) in the Cygnus X region in the two fine structure lines of
atomic carbon CI and four mid- transitions of CO and CO, and CS
J=7\TO6. These observations have been combined with FCRAO J=1\TO0
observations of CO and CO. Five positions, including DR21, DR21
(OH), and DR21 FIR1, were observed with the ISO/LWS grating spectrometer in the
\OI 63 and 145 m lines, the \CII 158 m line, and four high- CO
lines. We discuss the intensities and line ratios at these positions and apply
Local Thermal Equilibrium (LTE) and non-LTE analysis methods in order to derive
physical parameters such as masses, densities and temperatures. The CO line
emission has been modeled up to J=20. From non-LTE modeling of the low- to
high- CO lines we identify two gas components, a cold one at temperatures of
T_\RM{kin}\sim 30-40 K, and one with T_\RM{kin}\sim 80-150 K at a local
clump density of about n(H) cm. While the cold
quiescent component is massive containing typically more than 94 % of the mass,
the warm, dense, and turbulent gas is dominated by mid- and high- CO line
emission and its large line widths. The medium must be clumpy with a
volume-filling of a few percent. The CO lines are found to be important for the
cooling of the cold molecular gas, e.g. at DR21 (OH). Near the outflow of the
UV-heated source DR21, the gas cooling is dominated by line emission of atomic
oxygen and of CO
Structure of the outer layers of cool standard stars
Context: Among late-type red giants, an interesting change occurs in the
structure of the outer atmospheric layers as one moves to later spectral types
in the Hertzsprung-Russell diagram: a chromosphere is always present, but the
coronal emission diminishes and a cool massive wind steps in.
Aims: Where most studies have focussed on short-wavelength observations, this
article explores the influence of the chromosphere and the wind on
long-wavelength photometric measurements.
Methods: The observational spectral energy distributions are compared with
the theoretical predictions of the MARCS atmosphere models for a sample of 9 K-
and M-giants. The discrepancies found are explained using basic models for flux
emission originating from a chromosphere or an ionized wind.
Results: For 7 out of 9 sample stars, a clear flux excess is detected at
(sub)millimeter and/or centimeter wavelengths. The precise start of the excess
depends upon the star under consideration. The flux at wavelengths shorter than
about 1 mm is most likely dominated by an optically thick chromosphere, where
an optically thick ionized wind is the main flux contributor at longer
wavelengths.
Conclusions: Although the optical to mid-infrared spectrum of the studied K-
and M-giants is well represented by a radiative equilibrium atmospheric model,
the presence of a chromosphere and/or ionized stellar wind at higher altitudes
dominates the spectrum in the (sub)millimeter and centimeter wavelength ranges.
The presence of a flux excess also has implications on the role of these stars
as fiducial spectrophotometric calibrators in the (sub)millimeter and
centimeter wavelength range.Comment: 13 pages, 6 figures, 7 pages of online material, submitted to A&
A Self-Absorption Census of Cold HI Clouds in the Canadian Galactic Plane Survey
We present a 21cm line HI self-absorption (HISA) survey of cold atomic gas
within Galactic longitudes 75 to 146 degrees and latitudes -3 to +5 degrees. We
identify HISA as spatially and spectrally confined dark HI features and extract
it from the surrounding HI emission in the arcminute-resolution Canadian
Galactic Plane Survey (CGPS). We compile a catalog of the most significant
features in our survey and compare our detections against those in the
literature. Within the parameters of our search, we find nearly all previously
detected features and identify many new ones. The CGPS shows HISA in much
greater detail than any prior survey and allows both new and
previously-discovered features to be placed into the larger context of Galactic
structure. In space and radial velocity, faint HISA is detected virtually
everywhere that the HI emission background is sufficiently bright. This ambient
HISA population may arise from small turbulent fluctuations of temperature and
velocity in the neutral interstellar medium. By contrast, stronger HISA is
organized into discrete complexes, many of which follow a longitude-velocity
distribution that suggests they have been made visible by the velocity reversal
of the Perseus arm's spiral density wave. The cold HI revealed in this way may
have recently passed through the spiral shock and be on its way to forming
molecules and, eventually, new stars. This paper is the second in a series
examining HISA at high angular resolution. A companion paper (Paper III)
describes our HISA search and extraction algorithms in detail.Comment: 44 pages, including 13 figure pages; to appear in June 10 ApJ, volume
626; figure quality significantly reduced for astro-ph; for full resolution,
please see http://www.ras.ucalgary.ca/~gibson/hisa/cgps1_survey
The spectral energy distribution of D-type symbiotic stars: the role of dust shells
We have collected continuum data of a sample of D-type symbiotic stars. By
modelling their spectral energy distribution in a colliding-wind theoretical
scenario we have found the common characteristics to all the systems: 1) at
least two dust shells are clearly present, one at \sim 1000 K and the other at
\sim 400 K; they dominate the emission in the IR; 2) the radio data are
explained by thermal self-absorbed emission from the reverse shock between the
stars; while 3) the data in the long wavelength tail come from the expanding
shock outwards the system; 4) in some symbiotic stars, the contribution from
the WD in the UV is directly seen. Finally, 5) for some objects soft X-ray
emitted by bremsstrahlung downstream of the reverse-shock between the stars are
predicted. The results thus confirm the validity of the colliding wind model
and the important role of the shocks. The comparison of the fluxes calculated
at the nebula with those observed at Earth reveals the distribution throughout
the system of the different components, in particular the nebulae and the dust
shells. The correlation of shell radii with the orbital period shows that
larger radii are found at larger periods. Moreover, the temperatures of the
dust shells regarding the sample are found at 1000 K and <=400 K, while, in the
case of late giants, they spread more uniformly throughout the same range.Comment: 14 pages, 7 figures, 5 tables. Accepted for publication in MNRA
A Sample of Candidate Radio Stars in FIRST and SDSS
We conduct a search for radio stars by combining radio and optical data from
the Faint Images of the Radio Sky at Twenty cm survey (FIRST) and the Sloan
Digital Sky Survey (SDSS). The faint limit of SDSS makes possible a homogeneous
search for radio emission from stars of low optical luminosity. We select a
sample of 112 candidate radio stars in the magnitude range
and with radio flux mJy, from about 7000 deg of sky. The
selection criteria are positional coincidence within 1\arcsec, radio and
optical point source morphology, and an SDSS spectrum classified as stellar.
The sample contamination is estimated by random matching to be ,
suggesting that at most a small fraction of the selected candidates are genuine
radio stars. Therefore, we rule out a very rare population of extremely
radio-loud stars: no more than 1.2 of every million stars in the magnitude
range stars has radio flux mJy. We investigate the
optical and radio colors of the sample to find candidates that show the largest
likelihood of being real radio stars. The significant outliers from the stellar
locus, as well as the magnetically active stars, are the best candidates for
follow-up radio observations. We conclude that, while the present wide-area
radio surveys are not sensitive enough to provide homogeneous samples of the
extremely rare radio stars, upcoming surveys which exploit the great
sensitivity of current and planned telescopes do have sufficient sensitivity
and will allow the properties of this class of object to be investigated in
detail.Comment: Online data available at
http://www.astro.washington.edu/users/akimball/radiocat/radiostars . Accepted
for publication in the Astrophysical Journal. (fixed formatting issue
X-ray and optical monitoring of the peculiar source 4U 1700+24/V934 Her
(Abridged) We report on ASCA and BeppoSAX observations of the X-ray source 4U
1700+24 and on (quasi-)simultaneous spectroscopy of its optical counterpart,
V934 Her, from the Loiano 1.5-meter telescope. Archival ROSAT and RXTE data as
well as the RXTE ASM light curve of 4U 1700+24 are also analyzed along with a
1985 EXOSAT pointing. The optical spectra are typical of a M2 III star; a
revised distance to the object of ~400 pc is inferred. While these spectra do
not show either any change between the two epochs or any peculiar feature, the
X-ray spectra reveal a complex and long-term variable shape, with a clear soft
excess. The X-ray spectral properties of the source are best described by a
thermal Comptonization spectrum plus a soft energy(<1 keV) excess, which can be
modeled with a blackbody emission with kT_BB ~ 1 keV; the latter component is
not detected at the lowest source flux levels. The ratio between the two
components varies substantially with the source flux. The X-ray emission from
the object appears to become harder as its luminosity increases, and the RXTE
data acquired during an outburst occurred during Fall 1997 display a hard tail
detected up to 100 keV. Apart from erratic shot-noise variability on timescales
of tens to thousands of seconds, no significant pulsations or QPOs are found
from the timing analysis of the X-ray light curves. With the new distance
determination, the 2-10 keV X-ray luminosity range spanned in the considered
observations lies between ~2x10^32 and ~1x10^34 erg/s. All this allows us to
suggest a scenario consisting of a wide binary system in which a neutron star
accretes matter from the wind of an M giant.Comment: 14 pages, 5 figures, to be published on Astronomy & Astrophysics,
Main Journa
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