125 research outputs found
A multi-scale, multi-wavelength source extraction method: getsources
We present a multi-scale, multi-wavelength source extraction algorithm called
getsources. Although it has been designed primarily for use in the far-infrared
surveys of Galactic star-forming regions with Herschel, the method can be
applied to many other astronomical images. Instead of the traditional approach
of extracting sources in the observed images, the new method analyzes fine
spatial decompositions of original images across a wide range of scales and
across all wavebands. It cleans those single-scale images of noise and
background, and constructs wavelength-independent single-scale detection images
that preserve information in both spatial and wavelength dimensions. Sources
are detected in the combined detection images by following the evolution of
their segmentation masks across all spatial scales. Measurements of the source
properties are done in the original background-subtracted images at each
wavelength; the background is estimated by interpolation under the source
footprints and overlapping sources are deblended in an iterative procedure. In
addition to the main catalog of sources, various catalogs and images are
produced that aid scientific exploitation of the extraction results. We
illustrate the performance of getsources on Herschel images by extracting
sources in sub-fields of the Aquila and Rosette star-forming regions. The
source extraction code and validation images with a reference extraction
catalog are freely available.Comment: 31 pages, 27 figures, to be published in Astronomy & Astrophysic
Kinematics and the origin of the internal structures in HL Tau jet (HH 151)
Knotty structures of Herbig-Haro jets are common phenomena, and knowing the
origin of these structures is essential for understanding the processes of jet
formation. Basically, there are two theoretical approaches: different types of
instabilities in stationary flow, and velocity variations in the flow. We
investigate the structures with different radial velocities in the knots of the
HL Tau jet as well as its unusual behaviour starting from 20 arcsec from the
source. Collation of radial velocity data with proper motion measurements of
emission structures in the jet of HL Tau makes it possible to understand the
origin of these structures and decide on the mechanism for the formation of the
knotty structures in Herbig-Haro flows. We present observations obtained with a
6 m telescope (Russia) using the SCORPIO camera with scanning Fabry-Perot
interferometer. Two epochs of the observations of the HL/XZ Tau region in
Halpha emission (2001 and 2007) allowed us to measure proper motions for high
and low radial velocity structures. The structures with low and high radial
velocities in the HL Tau jet show the same proper motion. The point where the
HL Tau jet bents to the north (it coincides with the trailing edge of so-called
knot A) is stationary, i.e. does not have any perceptible proper motion and is
visible in Halpha emission only. We conclude that the high- and low- velocity
structures in the HL Tau jet represent bow-shocks and Mach disks in the
internal working surfaces of episodic outflows. The bend of the jet and the
brightness increase starting some distance from the source coincides with the
observed stationary deflecting shock. The increase of relative surface
brightness of bow-shocks could be the result of the abrupt change of the
physical conditions of the ambient medium as well as the interaction of a
highly collimated flow and the side wind from XZ Tau.Comment: To be published in Astronomy and Astrophysic
An analysis of spectra in the Red Rectangle nebula
This paper presents an analysis of a series of spectra in the Red Rectangle
nebula. Only the reddest part of the spectra can safely be attributed to light
from the nebula, and indicates Rayleigh scattering by the gas, in conformity
with the large angles of scattering involved and the proximity of the star. In
the blue, light from HD44179, refracted or scattered in the atmosphere,
dominates the spectra. This paper questions the reliability of ground-based
observations of extended objects in the blue.Comment: 25 figure
Evolution of dust and ice features around FU Orionis objects
(abridged) We present spectroscopy data for a sample of 14 FUors and 2 TTauri
stars observed with the Spitzer Space Telescope or with the Infrared Space
Observatory (ISO). Based on the appearance of the 10 micron silicate feature we
define 2 categories of FUors. Objects showing the silicate feature in
absorption (Category 1) are still embedded in a dusty and icy envelope. The
shape of the 10 micron silicate absorption bands is compared to typical dust
compositions of the interstellar medium and found to be in general agreement.
Only one object (RNO 1B) appears to be too rich in amorphous pyroxene dust, but
a superposed emission feature can explain the observed shape. We derive optical
depths and extinction values from the silicate band and additional ice bands at
6.0, 6.8 and 15.2 micron. In particular the analysis of the CO_2 ice band at
15.2 micron allows us to search for evidence for ice processing and constrains
whether the absorbing material is physically linked to the central object or in
the foreground. For objects showing the silicate feature in emission (Category
2), we argue that the emission comes from the surface layer of accretion disks.
Analyzing the dust composition reveals that significant grain growth has
already taken place within the accretion disks, but no clear indications for
crystallization are present. We discuss how these observational results can be
explained in the picture of a young, and highly active accretion disk. Finally,
a framework is proposed as to how the two categories of FUors can be understood
in a general paradigm of the evolution of young, low-mass stars. Only one
object (Parsamian 21) shows PAH emission features. Their shapes, however, are
often seen toward evolved stars and we question the object's status as a FUor
and discuss other possible classifications.Comment: accepted for publication in ApJ; 63 pages preprint style including 8
tables and 24 figure
Will the starless cores in Chamaeleon I and III turn prestellar?
The nearby Chamaeleon molecular cloud complex is a good laboratory to study
the process of low-mass star formation since it consists of three clouds with
very different properties. Cha III does not show any sign of star formation,
while star formation has been very active in Cha I and may already be
finishing. Our goal is to determine whether star formation can proceed in Cha
III, and to compare the results to our recent survey of Cha I. We used the
Large APEX Bolometer Array (LABOCA) to map Cha III in dust continuum emission
at 870 micron. 29 sources are extracted from the map, all of them being
starless. The starless cores are found down to a visual extinction of 1.9 mag,
in marked contrast with other molecular clouds, including Cha I. Apart from
this difference, the Cha III starless cores share very similar properties with
those found in Cha I. At most two sources have a mass larger than the critical
Bonnor-Ebert mass, which suggests that the fraction of prestellar cores is very
low, even lower than in Cha I. Only the most massive sources are candidate
prestellar cores, in agreement with the correlation found earlier in the Pipe
nebula. The mass distribution of the 85 starless cores of Cha I and III that
are not candidate prestellar cores is consistent with a single power law down
to the 90% completeness limit, with an exponent close to the Salpeter value. A
fraction of the starless cores in Cha I and III may still grow in mass and
become gravitationally unstable. Based on predictions of numerical simulations
of turbulent molecular clouds, we estimate that at most 50% and 20% of the
starless cores of Cha I and III, respectively, may form stars. The LABOCA
survey reveals that Cha III, and Cha I to some extent too, is a prime target to
study the formation of prestellar cores, and thus the onset of star formation.
(abridged).Comment: Accepted for publication in A&A. 22 pages, 16 figures, 4 tables. A
version with high-resolution figures is available on request to the first
autho
Far-infrared observations of a massive cluster forming in the Monoceros R2 filament hub
We present far-infrared observations of Monoceros R2 (a giant molecular cloud at approximately 830 pc distance, containing several sites of active star formation), as observed at 70 ÎŒm, 160 ÎŒm, 250 ÎŒm, 350 ÎŒm, and 500 ÎŒm by the Photodetector Array Camera and Spectrometer (PACS) and Spectral and Photometric Imaging Receiver (SPIRE) instruments on the Herschel Space Observatory as part of the Herschel imaging survey of OB young stellar objects (HOBYS) Key programme. The Herschel data are complemented by SCUBA-2 data in the submillimetre range, and WISE and Spitzer data in the mid-infrared. In addition, C18O data from the IRAM 30-m Telescope are presented, and used for kinematic information. Sources were extracted from the maps with getsources, and from the fluxes measured, spectral energy distributions were constructed, allowing measurements of source mass and dust temperature. Of 177 Herschel sources robustly detected in the region (a detection with high signal-to-noise and low axis ratio at multiple wavelengths), including protostars and starless cores, 29 are found in a filamentary hub at the centre of the region (a little over 1% of the observed area). These objects are on average smaller, more massive, and more luminous than those in the surrounding regions (which together suggest that they are at a later stage of evolution), a result that cannot be explained entirely by selection effects. These results suggest a picture in which the hub may have begun star formation at a point significantly earlier than the outer regions, possibly forming as a result of feedback from earlier star formation. Furthermore, the hub may be sustaining its star formation by accreting material from the surrounding filaments
The role of Galactic H iiregions in the formation of filaments High-resolution submilimeter imaging of RCW 120 with ArTĂ©MiS
Context. Massive stars and their associated ionized (H ii) regions could play a key role in the formation and evolution of filaments that host star formation. However, the properties of filaments that interact with H ii regions are still poorly known.
Aims. To investigate the impact of H ii regions on the formation of filaments, we imaged the Galactic H ii region RCW 120 and its surroundings where active star formation takes place and where the role of ionization feedback on the star formation process has already been studied.
Methods. We used the large-format bolometer camera ArTéMiS on the APEX telescope and combined the high-resolution ArTéMiS data at 350 ”m and 450 ”m with Herschel-SPIRE/HOBYS data at 350 and 500 ”m to ensure good sensitivity to a broad range of spatial scales. This allowed us to study the dense gas distribution around RCW 120 with a resolution of 800 or 0.05 pc at a distance of 1.34 kpc.
Results. Our study allows us to trace the median radial intensity profile of the dense shell of RCW 120. This profile is asymmetric, indicating a clear compression from the H ii region on the inner part of the shell. The profile is observed to be similarly asymmetric on both lateral sides of the shell, indicating a homogeneous compression over the surface. On the contrary, the profile analysis of a radial filament associated with the shell, but located outside of it, reveals a symmetric profile, suggesting that the compression from the ionized region is limited to the dense shell. The mean intensity profile of the internal part of the shell is well fitted by a Plummerlike profile with a deconvolved Gaussian full width at half maximum (FWHM) of 0.09 pc, as observed for filaments in low-mass star-forming regions.
Conclusions. Using ArTĂ©MiS data combined with Herschel-SPIRE data, we found evidence for compression from the inner part of the RCW 120 ionized region on the surrounding dense shell. This compression is accompanied with a significant (factor 5) increase of the local column density. This study suggests that compression exerted by H ii regions may play a key role in the formation of filaments and may further act on their hosted star formation. ArTĂ©MiS data also suggest that RCW 120 might be a 3D ring, rather than a spherical structure
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The role of Galactic HII regions in the formation of filaments : High-resolution submilimeter imaging of RCW 120 with ArTĂ©MiS
Context. Massive stars and their associated ionized (HâŻII) regions could play a key role in the formation and evolution of filaments that host star formation. However, the properties of filaments that interact with HâŻII regions are still poorly known.
Aims. To investigate the impact of HâŻII regions on the formation of filaments, we imaged the Galactic HâŻII region RCW 120 and its surroundings where active star formation takes place and where the role of ionization feedback on the star formation process has already been studied.
Methods. We used the large-format bolometer camera ArTĂ©MiS on the APEX telescope and combined the high-resolution ArTĂ©MiS data at 350 and 450 ÎŒm with Herschel-SPIRE/HOBYS data at 350 and 500 ÎŒm to ensure good sensitivity to a broad range of spatial scales. This allowed us to study the dense gas distribution around RCW 120 with a resolution of 8âČâČ or 0.05 pc at a distance of 1.34 kpc.
Results. Our study allows us to trace the median radial intensity profile of the dense shell of RCW 120. This profile is asymmetric, indicating a clear compression from the HâŻII region on the inner part of the shell. The profile is observed to be similarly asymmetric on both lateral sides of the shell, indicating a homogeneous compression over the surface. On the contrary, the profile analysis of a radial filament associated with the shell, but located outside of it, reveals a symmetric profile, suggesting that the compression from the ionized region is limited to the dense shell. The mean intensity profile of the internal part of the shell is well fitted by a Plummer-like profile with a deconvolved Gaussian full width at half maximum of 0.09 pc, as observed for filaments in low-mass star-forming regions.
Conclusions. Using ArTĂ©MiS data combined with Herschel-SPIRE data, we found evidence for compression from the inner part of the RCW 120 ionized region on the surrounding dense shell. This compression is accompanied with a significant (factor 5) increase of the local column density. This study suggests that compression exerted by HâŻII regions may play a key role in the formation of filaments and may further act on their hosted star formation. ArTĂ©MiS data also suggest that RCW 120 might be a 3D ring, rather than a spherical structure
<i>Herschel</i>-HOBYS study of the earliest phases of high-mass star formation in NGC 6357
Aims: To constrain models of high-mass star formation it is important to identify the massive dense cores (MDCs) that are able to form high-mass star(s). This is one of the purposes of the Herschel/HOBYS key programme. Here, we carry out the census and characterise of the properties of the MDCs population of the NGC 6357 HâŻII region.
Methods: Our study is based on the Herschel/PACS and SPIRE 70â500 ÎŒm images of NGC 6357 complemented with (sub-)millimetre and mid-infrared data. We followed the procedure established by the Herschel/HOBYS consortium to extract ~0.1 pc massive dense cores using the getsources software. We estimated their physical parameters (temperatures, masses, luminosities) from spectral energy distribution (SED) fitting.
Results: We obtain a complete census of 23 massive dense cores, amongst which one is found to be IR-quiet and twelve are starless, representing very early stages of the star-formation process. Focussing on the starless MDCs, we have considered their evolutionary status, and suggest that only five of them are likely to form a high-mass star.
Conclusions: We find that, contrarily to the case in NGC 6334, the NGC 6357 region does not exhibit any ridge or hub features that are believed to be crucial to the massive star formation process. This study adds support for an empirical model in which massive dense cores and protostars simultaneously accrete mass from the surrounding filaments. In addition, the massive star formation in NGC 6357 seems to have stopped and the hottest stars in Pismis 24 have disrupted the filaments
Infrared Signatures of Protoplanetary Disk Evolution
We investigate the observational signatures of a straightforward evolutionary
scenario for protoplanetary disks in which the disk mass of small (50 micron)
particles decreases homologously with time, but the disk structure and stellar
parameters do not change. Our goal is to identify optimal infrared spectral
indicators of the existence of disks, their structure, and mass evolution that
may be tested with the upcoming SIRTF mission. We present simulated spectral
energy distributions and colors over a wide range of masses. The SED is most
sensitive to disk mass in the far-IR and longer wavelengths, which is already
known from millimeter and radio observations. As the disk mass decreases, the
excess emission of the disk over the stellar photosphere diminishes more
rapidly at the longest rather than at short wavelengths. At near-infrared
wavelengths, the disk remains optically thick to stellar radiation over a wide
range of disk mass, resulting in a slower decline in the SED in this spectral
regime. Therefore, near-IR excesses (K-L) provide a robust means of detecting
disks in star clusters down to 1E-7 solar masses, while the far-IR excess
probes the disk mass. Reducing the disk mass results in a clear progression in
color-color diagrams with low mass disks displaying the bluest colors. We
interpret color-color diagrams for Taurus-Auriga sources in the context of
decreasing disk mass.Comment: ApJ Accepte
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