240 research outputs found
Effects of X-ray irradiation and disc flaring on the [Ne ii] 12.8 μm emission from young stellar objects
The [Ne ii] fine-structure emission line at 12.8 μm has been detected in several young stellar objects spectra. This line is thought to be produced by X-ray irradiation of the warm protoplanetary disc atmospheres; however, the observational correlation between [Ne ii] luminosities and measured X-ray luminosities shows a large scatter. Such spread limits the utility of this line as a probe of the gaseous phase of discs, as several authors have suggested pollution by outflows as a probable cause of the observed scatter. In this work, we explore the possibility that the large variations in the observed [Ne ii] luminosity may be caused instead by different star-disc parameters. In particular we study the effects that the hardness of the irradiating source and the structure (flaring) of the disc have on the luminosity and spectral profile of the [Ne ii] 12.8 μm line. We find that varying these parameters can indeed cause up to an order of magnitude variation in the emission luminosities which may explain the scatter observed, although our models predict somewhat smaller luminosities than those recently reported by other authors who observed the line with the Spitzer Space Telescope. Our models also show that the hardness of the spectrum has only a limited (undetectable) effect on the line profiles, while changes in the flaring power of the disc significantly affect the size of the [Ne ii] emission region and, as a consequence, its line profile. In particular, we suggest that broad-line profiles centred on the stellar radial velocity may be indicative of flat discs seen at large inclination angle
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
Two Mass Distributions in the L 1641 Molecular Clouds: The Herschel Connection of Dense Cores and Filaments in Orion A
We present Herschel survey maps of the L 1641 molecular clouds in Orion A. We extracted both the filaments and dense cores in the region. We identified which of the dense sources are proto- or pre-stellar, and studied their association with the identified filaments. We find that although most (71%) of the pre-stellar sources are located on filaments there, is still a significant fraction of sources not associated with such structures. We find that these two populations (on and off the identified filaments) have distinctly different mass distributions. The mass distribution of the sources on the filaments is found to peak at 4 M_☉ and drives the shape of the core mass function (CMF) at higher masses, which we fit with a power law of the form dN/dlogM∝M^(–1.4 ± 0.4). The mass distribution of the sources off the filaments, on the other hand, peaks at 0.8 M_☉ and leads to a flattening of the CMF at masses lower than ~4 M_☉. We postulate that this difference between the mass distributions is due to the higher proportion of gas that is available in the filaments, rather than in the diffuse cloud
On the shape of the mass-function of dense clumps in the Hi-GAL fields. II. Using Bayesian inference to study the clump mass function
Context. Stars form in dense, dusty clumps of molecular clouds, but little is
known about their origin, their evolution and their detailed physical
properties. In particular, the relationship between the mass distribution of
these clumps (also known as the "clump mass function", or CMF) and the stellar
initial mass function (IMF), is still poorly understood. Aims. In order to
better understand how the CMF evolve toward the IMF, and to discern the "true"
shape of the CMF, large samples of bona-fide pre- and proto-stellar clumps are
required. Two such datasets obtained from the Herschel infrared GALactic Plane
Survey (Hi-GAL) have been described in paper I. Robust statistical methods are
needed in order to infer the parameters describing the models used to fit the
CMF, and to compare the competing models themselves. Methods. In this paper we
apply Bayesian inference to the analysis of the CMF of the two regions
discussed in Paper I. First, we determine the Bayesian posterior probability
distribution for each of the fitted parameters. Then, we carry out a
quantitative comparison of the models used to fit the CMF. Results. We have
compared the results from several methods implementing Bayesian inference, and
we have also analyzed the impact of the choice of priors and the influence of
various constraints on the statistical conclusions for the preferred values of
the parameters. We find that both parameter estimation and model comparison
depend on the choice of parameter priors. Conclusions. Our results confirm our
earlier conclusion that the CMFs of the two Hi-GAL regions studied here have
very similar shapes but different mass scales. Furthermore, the lognormal model
appears to better describe the CMF measured in the two Hi-GAL regions studied
here. However, this preliminary conclusion is dependent on the choice of
parameters priors.Comment: Submitted for publication to A&A on November 12, 2013. This paper
contains 11 pages and 7 figure
Oxygen depletion in giant planets with different formation histories
The atmospheric C/O ratio of exoplanets is widely used to constrain their formation. To guarantee that the C/O ratio provides robust information, we need to accurately quantify the amount of C and O in exoplanetary atmospheres. In the case of O, water and carbon monoxide are generally studied as the two key carriers. However, oxygen is a very reactive element and does not bind with carbon; depending on the temperature, it also binds to refractory elements. Estimating the amount of oxygen bound to refractory elements is therefore critical for unbiased estimates of the C/O ratio. In this work, we investigate the oxygen deficit due to refractory elements and its effects on the atmospheric C/O ratio of giant exoplanets as a function of their metallicity and equilibrium temperature. We model the composition of planetary atmospheres assuming chemical equilibrium and using as input physically justified elemental mixtures arising from detailed planet formation simulations. Our results show how the interplay between the atmospheric temperature and non-solar abundances of oxygen and refractory elements can sequester large fractions of oxygen, introducing significant biases in evaluating the C/O ratio when this effect is not accounted for. We apply our results to the case of Jupiter in the Solar System and show how the currently estimated water abundance points to a true oxygen abundance that is four times the solar one
Source extraction and photometry for the far-infrared and sub-millimeter continuum in the presence of complex backgrounds
(Abridged) We present a new method for detecting and measuring compact
sources in conditions of intense, and highly variable, fore/background. While
all most commonly used packages carry out the source detection over the signal
image, our proposed method builds from the measured image a "curvature" image
by double-differentiation in four different directions. In this way point-like
as well as resolved, yet relatively compact, objects are easily revealed while
the slower varying fore/background is greatly diminished. Candidate sources are
then identified by looking for pixels where the curvature exceeds, in absolute
terms, a given threshold; the methodology easily allows us to pinpoint
breakpoints in the source brightness profile and then derive reliable guesses
for the sources extent. Identified peaks are fit with 2D elliptical Gaussians
plus an underlying planar inclined plateau, with mild constraints on size and
orientation. Mutually contaminating sources are fit with multiple Gaussians
simultaneously using flexible constraints. We ran our method on simulated
large-scale fields with 1000 sources of different peak flux overlaid on a
realistic realization of diffuse background. We find detection rates in excess
of 90% for sources with peak fluxes above the 3-sigma signal noise limit; for
about 80% of the sources the recovered peak fluxes are within 30% of their
input values.Comment: Accepted on 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
A necklace of dense cores in the high-mass star forming region G35.20-0.74N: ALMA observations
The present study aims at characterizing the massive star forming region
G35.20N, which is found associated with at least one massive outflow and
contains multiple dense cores, one of them recently found associated with a
Keplerian rotating disk. We used ALMA to observe the G35.20N region in the
continuum and line emission at 350 GHz. The observed frequency range covers
tracers of dense gas (e.g. H13CO+, C17O), molecular outflows (e.g. SiO), and
hot cores (e.g. CH3CN, CH3OH). The ALMA 870 um continuum emission map reveals
an elongated dust structure (0.15 pc long and 0.013 pc wide) perpendicular to
the large-scale molecular outflow detected in the region, and fragmented into a
number of cores with masses 1-10 Msun and sizes 1600 AU. The cores appear
regularly spaced with a separation of 0.023 pc. The emission of dense gas
tracers such as H13CO+ or C17O is extended and coincident with the dust
elongated structure. The three strongest dust cores show emission of complex
organic molecules characteristic of hot cores, with temperatures around 200 K,
and relative abundances 0.2-2x10^(-8) for CH3CN and 0.6-5x10^(-6) for CH3OH.
The two cores with highest mass (cores A and B) show coherent velocity fields,
with gradients almost aligned with the dust elongated structure. Those velocity
gradients are consistent with Keplerian disks rotating about central masses of
4-18 Msun. Perpendicular to the velocity gradients we have identified a
large-scale precessing jet/outflow associated with core B, and hints of an
east-west jet/outflow associated with core A. The elongated dust structure in
G35.20N is fragmented into a number of dense cores that may form massive stars.
Based on the velocity field of the dense gas, the orientation of the magnetic
field, and the regularly spaced fragmentation, we interpret this elongated
structure as the densest part of a 1D filament fragmenting and forming massive
stars.Comment: 24 pages, 26 figures, accepted for publication in Astronomy and
Astrophysics (abstract modified to fit arXiv restrictions
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
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