195 research outputs found
Massive 70 micron quiet clumps I: evidence of embedded low/intermediate-mass star formation activity
Massive clumps, prior to the formation of any visible protostars, are the
best candidates to search for the elusive massive starless cores. In this work
we investigate the dust and gas properties of massive clumps selected to be 70
micron quiet, therefore good starless candidates. Our sample of 18 clumps has
masses 300 < M < 3000 M_sun, radius 0.54 < R < 1.00 pc, surface densities Sigma
> 0.05 g cm^-2 and luminosity/mass ratio L/M < 0.3. We show that half of these
70 micron quiet clumps embed faint 24 micron sources. Comparison with GLIMPSE
counterparts shows that 5 clumps embed young stars of intermediate stellar mass
up to ~5.5 M_sun. We study the clump dynamics with observations of N2H+ (1-0),
HNC (1-0) and HCO+ (1-0) made with the IRAM 30m telescope. Seven clumps have
blue-shifted spectra compatible with infall signatures, for which we estimate a
mass accretion rate 0.04 < M_dot < 2.0 x 10^-3 M_sun yr^-1, comparable with
values found in high-mass protostellar regions, and free-fall time of the order
of t_ff = 3 x 10^5 yr. The only appreciable difference we find between objects
with and without embedded 24 micron sources is that the infall rate appears to
increase from 24 micron dark to 24 micron bright objects. We conclude that all
70 micron quiet objects have similar properties on clump scales, independently
of the presence of an embedded protostar. Based on our data we speculate that
the majority, if not all of these clumps may already embed faint, low-mass
protostellar cores. If these clumps are to form massive stars, this must occur
after the formation of these lower mass stars.Comment: 44 pages, 11 Figures. Accepted for publication in MNRA
A companion candidate in the gap of the T Cha transitional disk
T Cha is a young star surrounded by a cold disk. The presence of a gap within
its disk, inferred from fitting to the spectral energy distribution, has
suggested on-going planetary formation. We observed T Cha in L' and K_s with
NAOS-CONICA, the adaptive optics system at the VLT, using sparse aperture
masking. We detected a source in the L' data at a separation of 62+-7 mas,
position angle of 78+-1 degrees, and a contrast of delta L' = 5.1+-0.2 mag. The
object is not detected in the Ks band data, which show a 3-sigma contrast limit
of 5.2 mag at the position of the detected L' source. For a distance of 108 pc,
the detected companion candidate is located at 6.7 AU from the primary, well
within the disk gap. If T Cha and the companion candidate are bound, the
comparison of the L' and Ks photometry with evolutionary tracks shows that the
photometry is inconsistent with any unextincted photosphere at the age and
distance of T Cha. The detected object shows a very red Ks-L' color for which a
possible explanation would be a significant amount of dust around it. This
would imply that the companion candidate is young, which would strengthen the
case for a physical companion, and moreover that the object would be in the
substellar regime, according to the Ks upper limit. Another exciting
possibility would be that this companion is a recently formed planet within the
disk. Additional observations are mandatory to confirm that the object is bound
and to properly characterize it.Comment: 4 pages, 4 figures; accepted for publication by A&
Multi-scale dynamics in star-forming regions: the interplay between gravity and turbulence
In the multi-scale view of the star formation process the material flows from
large molecular clouds down to clumps and cores. In this paradigm it is still
unclear if it is gravity or turbulence that drives the observed supersonic
non-thermal motions during the collapse, in particular in high-mass regions,
and at which scales gravity becomes eventually dominant over the turbulence of
the interstellar medium. To investigate this problem we have combined the
dynamics of a sample of 70 micron-quiet clumps, selected to cover a wide range
of masses and surface densities, with the dynamics of the parent filaments in
which they are embedded. We observe a continuous interplay between turbulence
and gravity, where the former creates structures at all scales and the latter
takes the lead when a critical value of the surface density is reached,
Sigma_th = 0.1 g cm^-2. In the densest filaments this transition can occur at
the parsec, or even larger scales, leading to a global collapse of the whole
region and most likely to the formation of the massive objects.Comment: Proceedings of the "Multi-line Diagnostics of the Interstellar
Medium" IRAM conference, Nice, Franc
Establishing the evolutionary timescales of the massive star formation process through chemistry
(Abridged) Understanding the details of the formation process of massive
(i.e. M<8-10M) stars is a long-standing problem in astrophysics. [...]
We present a method to derive accurate timescales of the different evolutionary
phases of the high-mass star formation process. We model a representative
number of massive clumps of the ATLASGAL-TOP100 sample which cover all the
evolutionary stages. The models describe an isothermal collapse and the
subsequent warm-up phase, for which we follow their chemical evolution. The
timescale of each phase is derived by comparing the results of the models with
the properties of the sources of the ATLASGAL-TOP100 sample, taking into
account the mass and luminosity of the clumps, and the column densities of
methyl acetylene (CHCCH), acetonitrile (CHCN), formaldehyde (HCO)
and methanol (CHOH). We find that the chosen molecular tracers are affected
by the thermal evolution of the clumps, showing steep ice evaporation gradients
from 10 to 10 AU during the warm-up phase. We succeed in reproducing
the observed column densities of CHCCH and CHCN, while HCO and
CHOH show a poorer agreement with the observed values. The total (massive)
star formation time is found to be yr, which is defined by
the timescales of the individual evolutionary phases of the ATLASGAL-TOP100
sample: yr for 70-m weak, yr for
mid-IR weak, yr for mid-IR bright and
yr for HII-regions phases. Our models, with an appropriate selection of
molecular tracers that can act as chemical clocks, allow to get robust
estimates of the duration of the individual phases of the high-mass star
formation process, with the advantage of being capable to include additional
tracers aimed at increasing the accuracy of the estimated timescales.Comment: Published on A&A (19 pages, 9 figures, 7 tables
Distance follow-up by a remote medical care centre improves adherence to CPAP in patients with obstructive sleep apnoea over the short and long term
Background: Adherence to continuous positive air pressure (CPAP) in patients with obstructive sleep apnoea (OSA) has remained invariably low over the last decades. Remote monitoring of the nocturnal CPAP treatment, within telemedicine (TM)-based follow-up programs, in these patients has been suggested as a potential tool to improve adherence and release the workload of sleep units. The aim of this study was therefore to assess whether a follow-up program carried out by a Remote Medical Care Centre (RMCC), outside the sleep unit, improves adherence to CPAP in the short and long term in patients with OSA. Methods: In this pilot protocol, we enrolled 37 patients starting CPAP in our Sleep Centre (SC). After three months of standard care in our SC, patients initiated a six-month remote follow-up carried out by the RMCC, functioning as an intermediary between patients and SC. Monthly reports and indication for face-to-face visits were sent to the SC for six months. After this period patients returned to usual care for one year. Results were compared with those obtained in 38 patients (controls) followed with usual care over the same time range. Results: Mean nightly use of CPAP increased from 3.2 ± 2.4 h pre-RMCC to 5.2 ± 1.9 h post-RMCC (p < 0.0001). Nights/month of CPAP use improved from 19.8 ± 9.2 to 25.2 ± 2.5 (p < 0.05) and nights/month with CPAP use >4 h from 12.5 ± 10 to 21.03 ± 8.9 (p < 0.05). This improvement remained stable after 12 months from the return of patients to usual care. No significant changes in CPAP use were observed in controls over the time. Conclusion: A six-month follow-up through a remote facility can significantly improve adherence to CPAP in the short and long term. This pilot study provides a solid base for the design of multicentre randomized trials focusing on new models which are able to increase the long-term efficacy of TM programs
Testing Larson's relationships in massive clumps
We tested the validity of the three Larson relations in a sample of 213
massive clumps selected from the Herschel Hi-GAL survey and combined with data
from the MALT90 survey of 3mm emission lines. The clumps have been divided in 5
evolutionary stages to discuss the Larson relations also as function of
evolution. We show that this ensemble does not follow the three Larson
relations, regardless of clump evolutionary phase. A consequence of this
breakdown is that the virial parameter dependence with mass (and
radius) is only a function of the gravitational energy, independent of the
kinetic energy of the system, and is not a good descriptor of
clump dynamics. Our results suggest that clumps with clear signatures of infall
motions are statistically indistinguishable from clumps with no such
signatures. The observed non-thermal motions are not necessarily ascribed to
turbulence acting to sustain the gravity, but they may be due to the
gravitational collapse at the clump scales. This seems particularly true for
the most massive (M1000 M) clumps in the sample, where also
exceptionally high magnetic fields may not be enough to stabilize the collapse.Comment: Accepted for publication in MNRA
<i>Herschel</i> observations of B1-bS and B1-bN: two first hydrostatic core candidates in the Perseus star-forming cloud
We report far-infrared Herschel observations obtained between 70 μm and 500 μm of two star-forming dusty condensations, [HKM99] B1-bS and [HKM99] B1-bN, in the B1 region of the Perseus star-forming cloud. In the western part of the Perseus cloud, B1-bS is the only source detected in all six PACS and SPIRE photometric bands, but it is not visible in the Spitzer map at 24 μm. B1-bN is clearly detected between 100 μm and 250 μm. We have fitted the spectral energy distributions of these sources to derive their physical properties, and find that a simple greybody model fails to reproduce the observed spectral energy distributions. At least a two-component model is required, consisting of a central source surrounded by a dusty envelope. The properties derived from the fit, however, suggest that the central source is not a Class 0 object. We then conclude that while B1-bS and B1-bN appear to be more evolved than a pre-stellar core, the best-fit models suggest that their central objects are younger than a Class 0 source. Hence, they may be good candidates to be examples of the first hydrostatic core phase. The projected distance between B1-bS and B1-bN is a few Jeans lengths. If their physical separation is close to this value, this pair would allow studying the mutual interactions between two forming stars at a very early stage of their evolution
Supervised machine learning on Galactic filaments. Revealing the filamentary structure of the Galactic interstellar medium
Context. Filaments are ubiquitous in the Galaxy, and they host star formation. Detecting them in a reliable way is therefore key towards our understanding of the star formation process.
Aims: We explore whether supervised machine learning can identify filamentary structures on the whole Galactic plane.
Methods: We used two versions of UNet-based networks for image segmentation. We used H2 column density images of the Galactic plane obtained with Herschel Hi-GAL data as input data. We trained the UNet-based networks with skeletons (spine plus branches) of filaments that were extracted from these images, together with background and missing data masks that we produced. We tested eight training scenarios to determine the best scenario for our astrophysical purpose of classifying pixels as filaments.
Results: The training of the UNets allows us to create a new image of the Galactic plane by segmentation in which pixels belonging to filamentary structures are identified. With this new method, we classify more pixels (more by a factor of 2 to 7, depending on the classification threshold used) as belonging to filaments than the spine plus branches structures we used as input. New structures are revealed, which are mainly low-contrast filaments that were not detected before. We use standard metrics to evaluate the performances of the different training scenarios. This allows us to demonstrate the robustness of the method and to determine an optimal threshold value that maximizes the recovery of the input labelled pixel classification.
Conclusions: This proof-of-concept study shows that supervised machine learning can reveal filamentary structures that are present throughout the Galactic plane. The detection of these structures, including low-density and low-contrast structures that have never been seen before, offers important perspectives for the study of these filaments
The Herschel view of the on-going star formation in the Vela-C molecular cloud
As part of the Herschel guaranteed time key program 'HOBYS', we present the
photometric survey of the star forming region Vela-C, one of the nearest sites
of low-to-high-mass star formation in the Galactic plane. Vela-C has been
observed with PACS and SPIRE in parallel mode between 70 um and 500 um over an
area of about 3 square degrees. A photometric catalogue has been extracted from
the detections in each band, using a threshold of 5 sigma over the local
background. Out of this catalogue we have selected a robust sub-sample of 268
sources, of which 75% are cloud clumps and 25% are cores. Their Spectral Energy
Distributions (SEDs) have been fitted with a modified black body function. We
classify 48 sources as protostellar and 218 as starless. For two further
sources, we do not provide a secure classification, but suggest they are Class
0 protostars.
From SED fitting we have derived key physical parameters. Protostellar
sources are in general warmer and more compact than starless sources. Both
these evidences can be ascribed to the presence of an internal source(s) of
moderate heating, which also causes a temperature gradient and hence a more
peaked intensity distribution. Moreover, the reduced dimensions of protostellar
sources may indicate that they will not fragment further. A virial analysis of
the starless sources gives an upper limit of 90% for the sources
gravitationally bound and therefore prestellar. We fit a power law N(logM) prop
M^-1.1 to the linear portion of the mass distribution of prestellar sources.
This is in between that typical of CO clumps and those of cores in nearby
star-forming regions. We interpret this as a result of the inhomogeneity of our
sample, which is composed of comparable fractions of clumps and cores.Comment: 9 pages, 7 figures, accepted by A&
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