90 research outputs found
The mass of dusty clumps with temperature and density structure
We consider a dusty clump in the two cases of spherical and cylindrical
symmetry to investigate the effect of temperature and density gradients on the
observed flux density. Conversely, we evaluate how the presence of such
gradients affects the calculation of the clump mass from the observed flux. We
provide the reader with approximate expressions relating flux density and mass
in the optically thick and thin limits, in the Rayleigh-Jeans regime, and
discuss the reliability of these expressions by comparing them to the outcome
of a numerical code. Finally, we present an application of our calculations to
three examples taken from the literature, which shows how the correction
introduced after taking into account temperature and density gradients may
affect our conclusions on the stability of the clumps
Calibration of evolutionary diagnostics in high-mass star formation
The evolutionary classification of massive clumps that are candidate
progenitors of high-mass young stars and clusters relies on a variety of
independent diagnostics based on observables from the near-infrared to the
radio. A promising evolutionary indicator for massive and dense
cluster-progenitor clumps is the L/M ratio between the bolometric luminosity
and the mass of the clumps. With the aim of providing a quantitative
calibration for this indicator we used SEPIA/APEX to obtain CH3C2H(12-11)
observations, that is an excellent thermometer molecule probing densities >
10^5 cm^-3 , toward 51 dense clumps with M>1000 solar masses, and uniformly
spanning -2 < Log(L/M) < 2.3.
We identify three distinct ranges of L/M that can be associated to three
distinct phases of star formation in massive clumps. For L/M <1 no clump is
detected in CH3C2H , suggesting an inner envelope temperature below 30K. For 1<
L/M < 10 we detect 58% of the clumps, with a temperature between 30 and 35 K
independently from the exact value of L/M; such clumps are building up
luminosity due to the formation of stars, but no star is yet able to
significantly heat the inner clump regions. For L/M> 10 we detect all the
clumps, with a gas temperature rising with Log(L/M), marking the appearance of
a qualitatively different heating source within the clumps; such values are
found towards clumps with UCHII counterparts, suggesting that the quantitative
difference in T - L/M behaviour above L/M >10 is due to the first appearance of
ZAMS stars in the clumps.Comment: Astrophysical Journal Letters, Accepte
A Double-Jet System in the G31.41+0.31 Hot Molecular Core
This work presents a detailed study of the gas kinematics towards the "Hot
Molecular Core" (HMC) G31.41+0.31 via multi-epoch VLBI observations of the H2O
22 GHz and CH3OH 6.7 GHz masers, and single-epoch VLBI of the OH 1.6 GHz
masers. Water masers present a symmetric spatial distribution with respect to
the HMC center, where two nearby (0.2" apart), compact, VLA sources (labeled
"A" and "B") are previously detected. The spatial distribution of a first group
of water masers, named "J1", is well fit with an elliptical profile, and the
maser proper motions mainly diverge from the ellipse center, with average speed
of 36 km s-1. These findings strongly suggest that the "J1" water maser group
traces the heads of a young (dynamical time of 1.3 10^3 yr), powerful (momentum
rate of ~0.2 M_sun yr-1 km s-1), collimated (semi-opening angle ~10 deg) jet
emerging from a MYSO located close (within 0.15") to the VLA source "B". Most
of the water features not belonging to "J1" present an elongated (about 2" in
size), NE--SW oriented (PA = 70 deg), S-shape distribution, which we denote
with the label "J2". The elongated distribution of the "J2" group and the
direction of motion, approximately parallel to the direction of elongation, of
most "J2" water masers suggests the presence of another collimated outflow,
emitted from a MYSO near the VLA source "A". The orientation of the "J2" jet
agrees well with that (PA = 68 deg) of the well-defined V_LSR gradient across
the HMC revealed by previous interferometric, thermal line observations.
Furthermore, the "J2" jet is powerful enough to sustain the large momentum
rate, 0.3 M_sun yr-1 km s-1, estimated assuming that the V_LSR gradient
represents a collimated outflow. These two facts lead us to favour the
interpretation of the V_LSR gradient across the G31.41+0.31 HMC in terms of a
compact and collimated outflow.Comment: 23 pages, 7 figures, accepted for publication in Astronomy &
Astrophysic
A 10- YSO with a Keplerian disk and a nonthermal radio jet
We previously observed the star-forming region G16.590.05 through
interferometric observations of both thermal and maser lines, and identified a
high-mass young stellar object (YSO) which is surrounded by an accretion disk
and drives a nonthermal radio jet. We performed high-angular-resolution (beam
FWHM ~0.15") 1.2-mm continuum and line observations towards G16.590.05 with
the Atacama Large Millimeter Array (ALMA). The main dust clump, with size
~10 au, is resolved into four relatively compact (diameter ~2000 au)
millimeter (mm) sources. The source harboring the high-mass YSO is the most
prominent in molecular emission. By fitting the emission profiles of several
unblended and optically thin transitions of CHOCH and CHOH, we
derived gas temperatures inside the mm-sources in the range 42--131 K, and
calculated masses of 1--5 . A well-defined Local Standard of Rest
velocity (Vlsr) gradient is detected in most of the high-density molecular
tracers at the position of the high-mass YSO, pinpointed by compact 22-GHz
free-free emission. This gradient is oriented along a direction forming a large
(~70 degree) angle with the radio jet, traced by elongated 13-GHz continuum
emission. The butterfly-like shapes of the P-V plots and the linear pattern of
the emission peaks of the molecular lines at high velocity confirm that this
Vlsr gradient is due to rotation of the gas in the disk surrounding the
high-mass YSO. The disk radius is ~500 au, and the Vlsr distribution along the
major axis of the disk is well reproduced by a Keplerian profile around a
central mass of 102 . The position of the YSO is offset by >~
0.1" from the axis of the radio jet and the dust emission peak. To explain this
displacement we argue that the high-mass YSO could have moved from the center
of the parental mm source owing to dynamical interaction with one or more
companions.Comment: 16 pages, 12 figures, accepted by Astronomy & Astrophysics, Main
Journa
Search for massive protostar candidates in the southern hemisphere: II. Dust continuum emission
In an ongoing effort to identify and study high-mass protostellar candidates
we have observed in various tracers a sample of 235 sources selected from the
IRAS Point Source Catalog, mostly with dec < -30 deg, with the SEST antenna at
millimeter wavelengths. The sample contains 142 Low sources and 93 High, which
are believed to be in different evolutionary stages. Both sub-samples have been
studied in detail by comparing their physical properties and morphologies.
Massive dust clumps have been detected in all but 8 regions, with usually more
than one clump per region. The dust emission shows a variety of complex
morphologies, sometimes with multiple clumps forming filaments or clusters. The
mean clump has a linear size of ~0.5 pc, a mass of ~320 Msolar for a dust
temperature Td=30 K, an H_2 density of 9.5E5 cm-3, and a surface density of 0.4
g cm-2. The median values are 0.4 pc, 102 Msolar, 4E4 cm-3, and 0.14 g cm-2,
respectively. The mean value of the luminosity-to-mass ratio, L/M ~99
Lsolar/Msolar, suggests that the sources are in a young, pre-ultracompact HII
phase. We have compared the millimeter continuum maps with images of the mid-IR
MSX emission, and have discovered 95 massive millimeter clumps non-MSX
emitters, either diffuse or point-like, that are potential prestellar or
precluster cores. The physical properties of these clumps are similar to those
of the others, apart from the mass that is ~3 times lower than for clumps with
MSX counterpart. Such a difference could be due to the potential prestellar
clumps having a lower dust temperature. The mass spectrum of the clumps with
masses above M ~100 Msolar is best fitted with a power-law dN/dM proportional
to M-alpha with alpha=2.1, consistent with the Salpeter (1955) stellar IMF,
with alpha=2.35.Comment: 83 pages, 10 figures, 3 tables. Accepted for publication by A&A. The
full paper, including Fig.2 with the maps of all the individual regions,
complete Tables 1 and 2 can be found at
http://www.arcetri.astro.it/~starform/publ2005.ht
In ricordo dell’ing. Gianni Tofani
È recentemente scomparso l'ing. Gianni Tofani, una figura di primo piano dell'Osservatorio Astrofisico di Arcetri, che ha dato un apporto fondamentale allo sviluppo della radioastronomia italiana. In questo contributo ne ripercorriamo brevemente l'esperienza professionale, senza dimenticare la sua profonda umanità.Prof. Gianni Tofani, a leading figure at the Arcetri Astrophysical Observatory, passed away a few months ago. Prof. Tofani played a fundamental role in the development of radioastronomy both in Florence and in Italy as a whole. This short essay seeks to highlight his main professional achievements
Infall of gas as the formation mechanism of stars up to 20 times more massive than the Sun
Theory predicts and observations confirm that low-mass stars (like the Sun)
in their early life grow by accreting gas from the surrounding material. But
for stars ~ 10 times more massive than the Sun (~10 M_sun), the powerful
stellar radiation is expected to inhibit accretion and thus limit the growth of
their mass. Clearly, stars with masses >10 M_sun exist, so there must be a way
for them to form. The problem may be solved by non-spherical accretion, which
allows some of the stellar photons to escape along the symmetry axis where the
density is lower. The recent detection of rotating disks and toroids around
very young massive stars has lent support to the idea that high-mass (> 8
M_sun) stars could form in this way. Here we report observations of an ammonia
line towards a high-mass star forming region. We conclude from the data that
the gas is falling inwards towards a very young star of ~20 M_sun, in line with
theoretical predictions of non-spherical accretion.Comment: 11 pages, 2 figure
Submillimeter Observations of The Isolated Massive Dense Clump IRAS 20126+4104
We used the CSO 10.4 meter telescope to image the 350 micron and 450 micron
continuum and CO J=6-5 line emission of the IRAS 20126+4104 clump. The
continuum and line observations show that the clump is isolated over a 4 pc
region and has a radius of ~ 0.5 pc. Our analysis shows that the clump has a
radial density profile propto r ^{-1.2} for r <~ 0.1 pc and has propto r^{-2.3}
for r >~ 0.1 pc which suggests the inner region is infalling, while the infall
wave has not yet reached the outer region. Assuming temperature gradient of
r^{-0.35}, the power law indices become propto r ^{-0.9} for r < ~0.1 pc and
propto r^{-2.0} for r >~ 0.1 pc. Based on a map of the flux ratio of
350micron/450micron, we identify three distinct regions: a bipolar feature that
coincides with the large scale CO bipolar outflow; a cocoon-like region that
encases the bipolar feature and has a warm surface; and a cold layer outside of
the cocoon region. The complex patterns of the flux ratio map indicates that
the clump is no longer uniform in terms of temperature as well as dust
properties. The CO emission near the systemic velocity traces the dense clump
and the outer layer of the clump shows narrow line widths (< ~3 km/s). The
clump has a velocity gradient of ~ 2 km/s pc^{-1}, which we interpret as due to
rotation of the clump, as the equilibrium mass (~ 200 Msun) is comparable to
the LTE mass obtained from the CO line. Over a scale of ~ 1 pc, the clump
rotates in the opposite sense with respect to the >~ 0.03 pc disk associated
with the (proto)star. This is one of four objects in high-mass and low-mass
star forming regions for which a discrepancy between the rotation sense of the
envelope and the core has been found, suggesting that such a complex kinematics
may not be unusual in star forming regions.Comment: 34 pages, 13 figures, Accepted for publication in the Ap
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