1,631 research outputs found
High Accretion Rate during Class 0 Phase due to External Trigger
Recent observations indicate that some class 0 sources have orders of
magnitude higher accretion rates than those of class I. We investigated the
conditions for the high accretion rates of some class 0 sources by numerical
calculations, modelling an external trigger. For no external trigger, we find
that the maximum value of the accretion rate is determined by the ratio
of the gravitational energy to the thermal one within a flat inner
region of the cloud core. The accretion rate reaches \sim 10^{-4} M_{\sun}
yr^{-1} if the cloud core has . For an external trigger we find
that the maximum value of the accretion rate is proportional to the momentum
given to the cloud core. The accretion rate reaches > 10^{-4} M_{\sun}
yr^{-1} with a momentum of \sim 0.1 M_{\sun} km s^{-1} when the initial
central density of the cloud core is . A comparison
between recent observational results for prestellar cores and our no triggered
collapse model indicates that the flat inner regions of typical prestellar
cores are not large enough to cause accretion rates of \sim 10^{-4} M_{\sun}
yr^{-1}. Our results show that the triggered collapse of the cloud core is
more preferable for the origin of the high accretion rates of class 0 sources
than no triggered collapse.Comment: 7 pages, 8 figures, accepted for publication in MNRA
A Survey for Low-mass Stars and Brown Dwarfs in the Upper-Scorpius OB Association
The Upper-Scorpius association is the OB association nearest to the Sun (145
pc). Its young age (5 Myr) makes it an ideal place to search for low-mass stars
and brown dwarfs, as these objects should be relatively bright. We have
performed a photometric search for the low-mass members of the association,
using the R, I, and Z filters. The completeness limit is I=18.5 and the
saturation limit is I=13. We obtain 138 candidate members, covering nearly the
entire M spectral type range. We find an excess of brown dwarf candidates over
the number predicted by a Miller-Scalo Initial Mass Function. In addition, we
have performed infrared imaging and low resolution optical spectroscopy of
selected candidates. We find that the infrared observations confirm the
spectral types obtained with the optical photometry. Furthermore, we find
H_alpha in emission in 20 of the 22 objects observed spectroscopically. As
H_alpha is an indicator of youth, we believe that these 20 objects may belong
to the association. One of them, UScoCTIO 128 has a very strong and constant
H_alpha line (equivalent width: -130 A), and its position in the color
magnitude diagram suggests that it is a brown dwarf with mass equal to 0.02
msun. Confirmation of this and the other candidates will have to wait for
higher resolution observations that can reveal spectroscopic mass indicators
like Li I and gravity indicators, such as K I and the subordinate lines of Na
I.Comment: 20 pages, 7 figures, 3 tables, accepted in the Astronomical Journa
The origin of runaway stars
Milli-arcsecond astrometry provided by Hipparcos and by radio observations
makes it possible to retrace the orbits of some of the nearest runaway stars
and pulsars to determine their site of origin. The orbits of the runaways AE
Aurigae and mu Columbae and of the eccentric binary iota Orionis intersect each
other about 2.5 Myr ago in the nascent Trapezium cluster, confirming that these
runaways were formed in a binary-binary encounter. The path of the runaway star
zeta Ophiuchi intersects that of the nearby pulsar PSR J1932+1059, about 1 Myr
ago, in the young stellar group Upper Scorpius. We propose that this neutron
star is the remnant of a supernova that occurred in a binary system which also
contained zeta Oph, and deduce that the pulsar received a kick velocity of
about 350 km/s in the explosion. These two cases provide the first specific
kinematic evidence that both mechanisms proposed for the production of runaway
stars, the dynamical ejection scenario and the binary-supernova scenario,
operate in nature.Comment: 5 pages, including 2 eps-figures and 1 table, submitted to the ApJ
Letters. The manuscript was typeset using aaste
Pulsar Parallaxes at 5 GHz with the Very Long Baseline Array
We present the first pulsar parallaxes measured with phase-referenced pulsar
VLBI observations at 5 GHz. Due to the steep spectra of pulsars, previous
astrometric measurements have been at lower frequencies. However, the strongest
pulsars can be observed at 5 GHz, offering the benefit of lower combined
ionospheric and tropospheric phase errors, which usually limit VLBI astrometric
accuracy. The pulsars B0329+54, B0355+54 and B1929+10 were observed for 7
epochs spread evenly over 2 years. For B0329+54, large systematic errors lead
to only an upper limit on the parallax (pi < 1.5 mas). A new proper motion and
parallax were measured for B0355+54 (pi = 0.91 +- 0.16 mas), implying a
distance of 1.04+0.21-0.16 kpc and a transverse velocity of 61+12-9 km/s. The
parallax and proper motion for B1929+10 were significantly improved (pi = 2.77
+- 0.07 mas), yielding a distance of 361+10-8 pc and a transverse velocity of
177+4-5 km/s. We demonstrate that the astrometric errors are correlated with
the angular separation between the phase reference calibrator and the target
source, with significantly lower errors at 5 GHz compared to 1.6 GHz. Finally,
based on our new distance determinations for B1929+10 and B0355+54, we derive
or constrain the luminosities of each pulsar at high energies. We show that,
for thermal emission models, the emitting area for X-rays from PSR B1929+10 is
roughly consistent with the canonical size for a heated polar cap, and that the
conversion of spin-down power to gamma-ray luminosity in B0355+54 must be low.
The new proper motion for B1929+10 also implies that its progenitor is unlikely
to have been the binary companion of the runaway O-star zeta-Ophiuchi.Comment: 8 pages, including 3 figures and 3 tables; emulateapj; ApJ submitte
Photometric Accretion Signatures Near the Substellar Boundary
Multi-epoch imaging of the Orion equatorial region by the Sloan Digital Sky
Survey has revealed that significant variability in the blue continuum persists
into the late-M spectral types, indicating that magnetospheric accretion
processes occur below the substellar boundary in the Orion OB1 association. We
investigate the strength of the accretion-related continuum veiling by
comparing the reddening-invariant colors of the most highly variable stars
against those of main sequence M dwarfs and evolutionary models. A gradual
decrease in the g band veiling is seen for the cooler and less massive members,
as expected for a declining accretion rate with decreasing mass. We also see
evidence that the temperature of the accretion shock decreases in the very low
mass regime, reflecting a reduction in the energy flux carried by the accretion
columns. We find that the near-IR excess attributed to circumstellar disk
thermal emission drops rapidly for spectral types later than M4. This is likely
due to the decrease in color contrast between the disk and the cooler stellar
photosphere. Since accretion, which requires a substantial stellar magnetic
field and the presence of a circumstellar disk, is inferred for masses down to
0.05 Msol we surmise that brown dwarfs and low mass stars share a common mode
of formation.Comment: 37 pages, 14 figures, accepted by A
Heritability of reproductive hormones in adult male twins
Background: Proper functioning of the male reproductive axis depends on complex feedback systems between several hormones. In this study, the genetic contribution of various endocrine components of the hypothalamic-pituitary-testicular axis is evaluated and previously observed differences in FSH and inhibin B levels between mono- (MZ) and dizygotic (DZ) twins are re-investigated. Methods: Inhibin B, FSH, LH, sex hormone-binding globulin (SHBG) and testosterone levels were assayed in 128 adult males (20 MZ twin pairs, 7 single MZ twins, 10 DZ twin pairs, 27 single DZ twins and 34 siblings of twins, constituting 10 sibling pairs), aged 15.6-68.7 years. Hormone levels were compared across zygosity groups and heritability estimates were obtained using maximum likelihood variance component analysis. Results: Heritability estimates ranged from 56% (testosterone) to 81% (inhibin B and SHBG). For LH and FSH, the heritability was estimated at 68% and 80% respectively. No mean differences in hormone levels were observed across groups. Conclusions: All measured hormones are highly heritable. A difference in the FSH-inhibin B feedback system between DZ twin males and MZ twin males could not be confirmed. © The Author 2007. Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology. All rights reserved
Cloud Structure and Physical Conditions in Star-Forming Regions from Optical Observations. II. Analysis
To complement the optical absorption-line survey of diffuse molecular gas in
Paper I, we obtained and analyzed far ultraviolet H and CO data on lines of
sight toward stars in Cep OB2 and Cep OB3. Possible correlations between column
densities of different species for individual velocity components, not total
columns along a line of sight as in the past, were examined and were
interpreted in terms of cloud structure. The analysis reveals that there are
two kinds of CH in diffuse molecular gas: CN-like CH and CH-like CH.
Evidence is provided that CO is also associated with CN in diffuse molecular
clouds. Different species are distributed according to gas density in the
diffuse molecular gas. Both calcium and potassium may be depleted onto grains
in high density gas, but with different dependences on local gas density. Gas
densities for components where CN was detected were inferred from a chemical
model. Analysis of cloud structure indicates that our data are generally
consistent with the large-scale structure suggested by maps of CO
millimeter-wave emission. On small scales, the gas density is seen to vary by
factors greater than 5.0 over scales of 10,000 AU. The relationships
between column densities of CO and CH with that of H along a line of sight
show similar slopes for the gas toward Cep OB2 and OB3, but the CO/H and
CH/H ratios tend to differ which we ascribe to variation in average density
along the line of sight.Comment: 49 pages, 9 figures, accepted by Ap
Tracing the Mass during Low-Mass Star Formation. II. Modelling the Submillimeter Emission from Pre-Protostellar Cores
We have modeled the emission from dust in pre-protostellar cores, including a
self-consistent calculation of the temperature distribution for each input
density distribution. Model density distributions include Bonnor-Ebert spheres
and power laws. The Bonnor-Ebert spheres fit the data well for all three cores
we have modeled. The dust temperatures decline to very low values (\Td \sim 7
K) in the centers of these cores, strongly affecting the dust emission.
Compared to earlier models that assume constant dust temperatures, our models
indicate higher central densities and smaller regions of relatively constant
density. Indeed, for L1544, a power-law density distribution, similar to that
of a singular, isothermal sphere, cannot be ruled out. For the three sources
modeled herein, there seems to be a sequence of increasing central
condensation, from L1512 to L1689B to L1544. The two denser cores, L1689B and
L1544, have spectroscopic evidence for contraction, suggesting an evolutionary
sequence for pre-protostellar cores.Comment: 22 pages, 9 figures, Ap. J. accepted, uses emulateapj5.st
The CO Molecular Outflows of IRAS 16293-2422 Probed by the Submillimeter Array
We have mapped the proto-binary source IRAS 16293-2422 in CO 2-1, 13CO 2-1,
and CO 3-2 with the Submillimeter Array (SMA). The maps with resolution of
1".5-5" reveal a single small scale (~3000 AU) bipolar molecular outflow along
the east-west direction. We found that the blueshifted emission of this small
scale outflow mainly extends to the east and the redshifted emission to the
west from the position of IRAS 16293A. A comparison with the morphology of the
large scale outflows previously observed by single-dish telescopes at
millimeter wavelengths suggests that the small scale outflow may be the inner
part of the large scale (~15000 AU) E-W outflow. On the other hand, there is no
clear counterpart of the large scale NE-SW outflow in our SMA maps. Comparing
analytical models to the data suggests that the morphology and kinematics of
the small scale outflow can be explained by a wide-angle wind with an
inclination angle of ~30-40 degrees with respect to the plane of the sky. The
high resolution CO maps show that there are two compact, bright spots in the
blueshifted velocity range. An LVG analysis shows that the one located 1" to
the east of source A is extremely dense, n(H_2)~10^7 cm^-3, and warm, T_kin >55
K. The other one located 1" southeast of source B has a higher temperature of
T_kin >65 K but slightly lower density of n(H_2)~10^6 cm^-3. It is likely that
these bright spots are associated with the hot core-like emission observed
toward IRAS 16293. Since both two bright spots are blueshifted from the
systemic velocity and are offset from the protostellar positions, they are
likely formed by shocks.Comment: 27 pages, 8 figures, accepted for publication in ApJ, minor typos
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