2,838 research outputs found
The Impact of Feedback in Massive Star Formation. II. Lower Star Formation Efficiency at Lower Metallicity
We conduct a theoretical study of the formation of massive stars over a wide
range of metallicities from 1e-5 to 1Zsun and evaluate the star formation
efficiencies (SFEs) from prestellar cloud cores taking into account multiple
feedback processes. Unlike for simple spherical accretion, in the case of disk
accretion feedback processes do not set upper limits on stellar masses. At
solar metallicity, launching of magneto-centrifugally-driven outflows is the
dominant feedback process to set SFEs, while radiation pressure, which has been
regarded to be pivotal, has only minor contribution even in the formation of
over-100Msun stars. Photoevaporation becomes significant in over-20Msun star
formation at low metallicities of <1e-2Zsun, where dust absorption of ionizing
photons is inefficient. We conclude that if initial prestellar core properties
are similar, then massive stars are rarer in extremely metal-poor environments
of 1e-5 - 1e-3Zsun. Our results give new insight into the high-mass end of the
initial mass function and its potential variation with galactic and
cosmological environments.Comment: 13 pages, 9 figures, accepted for publication in The Astrophysical
Journa
The Formation and Destruction of Molecular Clouds and Galactic Star Formation
We describe an overall picture of galactic-scale star formation. Recent
high-resolution magneto-hydrodynamical simulations of two-fluid dynamics with
cooling/heating and thermal conduction have shown that the formation of
molecular clouds requires multiple episodes of supersonic compression. This
finding enables us to create a scenario in which molecular clouds form in
interacting shells or bubbles on a galactic scale. First we estimate the
ensemble-averaged growth rate of molecular clouds over a timescale larger than
a million years. Next we perform radiation hydrodynamics simulations to
evaluate the destruction rate of magnetized molecular clouds by the stellar FUV
radiation. We also investigate the resultant star formation efficiency within a
cloud which amounts to a low value (a few percent) if we adopt the power-law
exponent -2.5 for the mass distribution of stars in the cloud. We finally
describe the time evolution of the mass function of molecular clouds over a
long timescale (>1Myr) and discuss the steady state exponent of the power-law
slope in various environments.Comment: 7 pages, 3 figures. Accepted for publication in Astronomy and
Astrophysic
Episodic excursions of low-mass protostars on the Hertzsprung-Russell diagram
Following our recent work devoted to the effect of accretion on the
pre-main-sequence evolution of low-mass stars, we perform a detailed analysis
of episodic excursions of low-mass protostars in the Hertzsprung-Russell (H-R)
diagram triggered by strong mass accretion bursts typical of FU Orionis-type
objects (FUors). These excursions reveal themselves as sharp increases in the
stellar total luminosity and/or effective temperature of the protostar and can
last from hundreds to a few thousands of years, depending on the burst strength
and characteristics of the protostar. During the excursions, low-mass
protostars occupy the same part of the H-R diagram as young intermediate-mass
protostars in the quiescent phase of accretion. Moreover, the time spent by
low-mass protostars in these regions is on average a factor of several longer
than that spent by the intermediate-mass stars in quiescence. During the
excursions, low-mass protostars pass close to the position of most known FUors
in the H-R diagram, but owing to intrinsic ambiguity the model stellar
evolutionary tracks are unreliable in determining the FUor properties. We find
that the photospheric luminosity in the outburst state may dominate the
accretion luminosity already after a few years after the onset of the outburst,
meaning that the mass accretion rates of known FUors inferred from the
bolometric luminosity may be systematically overestimated, especially in the
fading phase.Comment: 15 pages, 12 figure
Toward the Stable Optical Trapping of a Droplet with Counter Laser Beams under Microgravity
To identify the optimum conditions for the optical trapping of a droplet
under microgravity, we theoretically analyzed the efficiency of trapping with
counter laser beams. We found that the distance between the two foci is an
important parameter for obtaining stable trapping conditions. We also performed
an optical trapping experiment with counter laser beams under microgravity. The
experimental results correspond well to the theoretical prediction
Direct diagnostics of forming massive stars: stellar pulsation and periodic variability of maser sources
The 6.7 GHz methanol maser emission, a tracer of forming massive stars,
sometimes shows enigmatic periodic flux variations over several 10-100 days. In
this Letter, we propose that this periodic variations could be explained by the
pulsation of massive protostars growing under rapid mass accretion with rates
of Mdot > 10^-3 Msun/yr. Our stellar evolution calculations predict that the
massive protostars have very large radius exceeding 100 Rsun at maximum, and we
here study the pulsational stability of such the bloated protostars by way of
the linear stability analysis. We show that the protostar becomes pulsationally
unstable with various periods of several 10-100 days, depending on different
accretion rates. With the fact that the stellar luminosity when the star is
pulsationally unstable also depends on the accretion rate, we derive the
period-luminosity relation log (L/Lsun) = 4.62 + 0.98log(P/100 day), which is
testable with future observations. Our models further show that the radius and
mass of the pulsating massive protostar should also depend on the period. It
would be possible to infer such protostellar properties and the accretion rate
with the observed period. Measuring the maser periods enables a direct
diagnosis of the structure of accreting massive protostars, which are deeply
embedded in dense gas and inaccessible with other observations.Comment: 5 pages, 3 figures, 1 table, accepted for publication in ApJ
The spin axes orbital alignment of both stars within the eclipsing binary system V1143Cyg using the Rossiter-McLaughlin effect
Context: The Rossiter-McLaughlin (RM) effect, a rotational effect in
eclipsing systems, provides unique insight into the relative orientation of
stellar spin axes and orbital axes of eclipsing binary systems.
Aims: Our aim is to develop a robust method to analyze the RM effect in an
eclipsing system with two nearly equally bright components. This gives access
to the orientation of the stellar rotation axes and may shed light on questions
of binary formation and evolution.
Methods: High-resolution spectra have been obtained both out of eclipse and
during the primary and secondary eclipses in the V1143Cyg system, using the
high-resolution Hamilton Echelle Spectrograph at the Lick Observatory. The
Rossiter-McLaughlin effect is analyzed in two ways: (1) by measuring the shift
of the line center of gravity during different phases of the eclipses and (2)
by analysis of the line shape change of the rotational broadening function
during eclipses.
Results: The projected axes of both stars are aligned with the orbital spin
within the observational uncertainties, with the angle of the primary rotation
axis beta_p=0.3+-1.5 deg, and the angle of the secondary rotation axis
beta_s=-1.2+-1.6 deg, thereby showing that the remaining difference between the
theoretical and observed apsidal motion for this system is not due to a
misalignment of the stellar rotation axes. Both methods utilized in this paper
work very well, even at times when the broadening profiles of the two stars
overlap.[abridged]Comment: Accepted for publication in A&A; 11 pages, 9 figures, 3 tables ; a
typo in the abstract has been correcte
Observational signatures of forming young massive clusters: continuum emission from dense HII regions
Young massive clusters (YMCs) are the most massive star clusters forming in
nearby galaxies and are thought to be a young analogue to the globular
clusters. Understanding the formation process of YMCs leads to looking into
very efficient star formation in high-redshift galaxies suggested by recent
JWST observations. We investigate possible observational signatures of their
formation stage, particularly when the mass of a cluster is increasing via
accretion from a natal molecular cloud. To this end, we study the broad-band
continuum emission from ionized gas and dust enshrouding YMCs, whose formation
is followed by recent radiation-hydrodynamics simulations. We perform
post-process radiative transfer calculations using simulation snapshots and
find characteristic spectral features at radio and far-infrared frequencies. We
show that a striking feature is long-lasting, strong free-free emission from a
10pc-scale HII region with a large emission measure of , corresponding to the mean electron density of
. There is a turnover feature below 10
GHz, a signature of the optically-thick free-free emission, often found in
Galactic ultra-compact HII regions. These features come from the peculiar YMC
formation process, where the cluster's gravity effectively traps photoionized
gas for a long duration and enables continuous star formation within the
cluster. Such large and dense HII regions show distinct distribution on the
density-size diagram, apart from the standard sequence of Galactic HII regions.
This is consistent with the observational trend inferred for extragalactic HII
regions associated with YMCs.Comment: 12 pages, 10 figures, accepted for publication in MNRA
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