747 research outputs found
Kinetic energy change with doping upon superfluid condensation in high temperature superconductors
In conventional BCS superconductors, the electronic kinetic energy increases
upon superfluid condensation (the change DEkin is positive). Here we show that
in the high critical temperature superconductor Bi-2212, DEkin crosses over
from a fully compatible conventional BCS behavior (DEkin>0) to an
unconventional behavior (DEkin<0) as the free carrier density decreases. If a
single mechanism is responsible for superconductivity across the whole phase
diagram of high critical temperature superconductors, this mechanism should
allow for a smooth transition between such two regimes around optimal doping.Comment: 3 pages, 2 figure
Infrared Photometry of Starless Dense Cores
Deep JHKs photometry was obtained towards eight dense molecular cores and J-H
vs. H-Ks color-color plots are presented. Our photometry, sensitive to the
detection of a 1 solar mass, 1 X 10^6 year old star through approx. 35 - 50
magnitudes of visual extinction, shows no indication of the presence of
star/disk systems based on J-H vs. H-Ks colors of detected objects. The stars
detected towards the cores are generally spatially anti-correlated with core
centers suggesting a background origin, although we cannot preclude the
possibility that some stars detected at H and Ks alone, or Ks alone, are not
low mass stars or brown dwarfs (< 0.3 Solar Masses) behind substantial amounts
of visual extinction (e.g. 53 magnitudes for L183B). Lower limits to optical
extinctions are estimated for the detected background stars, with high
extinctions being encountered, in the extreme case ranging up to at least Av =
46, and probably higher. The extinction data are used to estimate cloud masses
and densities which are comparable to those determined from molecular line
studies. Variations in cloud extinctions are consistent with a systematic
nature to cloud density distributions and column density variations and
extinctions are found to be consistent with submillimeter wave continuum
studies of similar regions. The results suggest that some cores have achieved
significant column density contrasts (approx. 30) on sub-core scales (approx.
0.05 pc) without having formed known stars.Comment: 44 pages including tables and figures, accepted ApJ, March 24, 200
Do Proto-Jovian Planets Drive Outflows?
We discuss the possibility that gaseous giant planets drive strong outflows
during early phases of their formation. We consider the range of parameters
appropriate for magneto-centrifugally driven stellar and disk outflow models
and find that if the proto-Jovian planet or accretion disk had a magnetic field
of >~ 10 Gauss and moderate mass inflow rates through the disk of less than
10^-7 M_J/yr that it is possible to drive an outflow. Estimates based both on
scaling from empirical laws observed in proto-stellar outflows and the
magneto-centrigugal disk and stellar+disk wind models suggest that winds with
mass outflow rates of 10^-8 M_J/yr and velocities of order ~ 20 km/s could be
driven from proto-Jovian planets. Prospects for detection and some implications
for the formation of the solar system are briefly discussed.Comment: AAS Latex, accepted for Ap
Infall models of Class 0 protostars
We have carried out radiative transfer calculations of infalling, dusty
envelopes surrounding embedded protostars to understand the observed properties
of the recently identified ``Class 0'' sources. To match the far-infrared peaks
in the spectral energy distributions of objects such as the prototype Class 0
source VLA 1623, pure collapse models require mass infall rates
\sim10^{-4}\msunyr. The radial intensity distributions predicted by
such infall models are inconsistent with observations of VLA 1623 at sub-mm
wavelengths, in agreement with the results of Andre et al. (1993) who found a
density profile of rather than the expected gradient. To resolve this conflict, while still invoking
infall to produce the outflow source at the center of VLA 1623, we suggest that
the observed sub-mm intensity distribution is the sum of two components: an
inner infall zone, plus an outer, more nearly constant-density region. This
explanation of the observations requires that roughly half the total mass
observed within 2000 AU radius of the source lies in a region external to the
infall zone. The column densities for this external region are comparable to
those found in the larger Oph A cloud within which VLA 1623 is embedded. The
extreme environments of Class 0 sources lead us to suggest an alternative or
additional interpretation of these objects: rather than simply concluding with
Andre et al. that Class 0 objects only represent the earliest phases of
protostellar collapse, and ultimately evolve into older ``Class I'' protostars,
we suggest that many Class 0 sources could be the protostars of very dense
regions. (Shortened)Comment: 22 pages, including 3 PostScript figures, accepted for publication in
The Astrophysical Journa
Molecular Tracers of Embedded Star Formation in Ophiuchus
In this paper we analyze nine SCUBA cores in Ophiuchus using the
second-lowest rotational transitions of four molecular species (12CO, 13CO,
C18O, and C17O) to search for clues to the evolutionary state and
star-formation activity within each core. Specifically, we look for evidence of
outflows, infall, and CO depletion. The line wings in the CO spectra are used
to detect outflows, spectral asymmetries in 13CO are used to determine infall
characteristics, and a comparison of the dust emission (from SCUBA
observations) and gas emission (from C18O) is used to determine the fractional
CO freeze-out.
Through comparison with Spitzer observations of protostellar sources in
Ophiuchus, we discuss the usefulness of CO and its isotopologues as the sole
indicators of the evolutionary state of each core. This study is an important
pilot project for the JCMT Legacy Survey of the Gould Belt (GBS) and the
Galactic Plane (JPS), which intend to complement the SCUBA-2 dust continuum
observations with HARP observations of 12CO, 13CO, C18O, and C17O J = 3 - 2 in
order to determine whether or not the cold dust clumps detected by SCUBA-2 are
protostellar or starless objects.
Our classification of the evolutionary state of the cores (based on molecular
line maps and SCUBA observations) is in agreement with the Spitzer designation
for six or seven of the nine SCUBA cores. However, several important caveats
exist in the interpretation of these results, many of which large mapping
surveys like the GBS may be able to overcome to provide a clearer picture of
activity in crowded fields.Comment: 43 pages including 19 postscript figures. Accepted for publication in
the PAS
Star formation triggered by the Galactic HII region RCW 120: First results from the Herschel Space Observatory
By means of different physical mechanisms, the expansion of HII regions can
promote the formation of new stars of all masses. RCW 120 is a nearby Galactic
HII region where triggered star formation occurs. This region is well-studied -
there being a wealth of existing data - and is nearby. However, it is
surrounded by dense regions for which far infrared data is essential to obtain
an unbiased view of the star formation process and in particular to establish
whether very young protostars are present. We attempt to identify all Young
Stellar Objects (YSOs), especially those previously undetected at shorter
wavelengths, to derive their physical properties and obtain insight into the
star formation history in this region. We use Herschel-PACS and -SPIRE images
to determine the distribution of YSOs observed in the field. We use a spectral
energy distribution fitting tool to derive the YSOs physical properties.
Herschel-PACS and -SPIRE images confirm the existence of a young source and
allow us to determine its nature as a high-mass (8-10 MSun) Class 0 object
(whose emission is dominated by a massive envelope) towards the massive
condensation 1 observed at (sub)-millimeter wavelengths. This source was not
detected at 24 micron and only barely seen in the MISPGAL 70 micron data.
Several other red sources are detected at Herschel wavelengths and coincide
with the peaks of the millimeter condensations. SED fitting results for the
brightest Herschel sources indicate that, apart from the massive Class 0 that
forms in condensation 1, young low mass stars are forming around RCW 120. The
YSOs observed on the borders of RCW 120 are younger than its ionizing star,
which has an age of about 2.5 Myr.Comment: 5 pqges, 3 figures, accepted by A&A (Special issue on the Herschel
first results
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
Tracing the Mass during Low-Mass Star Formation. III. Models of the Submillimeter Dust Continuum Emission from Class 0 Protostars
Seven Class 0 sources mapped with SCUBA at 850 and 450 micron are modeled
using a one dimensional radiative transfer code. The modeling takes into
account heating from an internal protostar, heating from the ISRF, realistic
beam effects, and chopping to model the normalized intensity profile and
spectral energy distribution. Power law density models, n(r) ~ r^{-p}, fit all
of the sources; best fit values are mostly p = 1.8 +/- 0.1, but two sources
with aspherical emission contours have lower values (p ~ 1.1). Including all
sources, = 1.63 +/- 0.33. Based on studies of the sensitivity of the
best-fit p to variations in other input parameters, uncertainties in p for an
envelope model are \Delta p = +/- 0.2. If an unresolved source (e.g., a disk)
contributes 70% of the flux at the peak, p is lowered in this extreme case and
\Delta p = ^{+0.2}_{-0.6}. The models allow a determination of the internal
luminosity ( = 4.0 \lsun) of the central protostar as well as a
characteristic dust temperature for mass determination ( = 13.8 +/-
2.4 K). We find that heating from the ISRF strongly affects the shape of the
dust temperature profile and the normalized intensity profile, but does not
contribute strongly to the overall bolometric luminosity of Class 0 sources.
There is little evidence for variation in the dust opacity as a function of
distance from the central source. The data are well-fitted by dust opacities
for coagulated dust grains with ice mantles (Ossenkopf & Henning 1994). The
density profile from an inside-out collapse model (Shu 1977) does not fit the
data well, unless the infall radius is set so small as to make the density
nearly a power-law.Comment: Accepted to ApJ. 28 pages, 13 figures, uses emulateapj5.st
Star Formation Near Photodissociation Regions: Detection of a Peculiar Protostar Near Ced 201
We present the detection and characterization of a peculiar low-mass
protostar (IRAS 22129+7000) located ~0.4 pc from Ced 201 Photodissociation
Region (PDR) and ~0.2 pc from the HH450 jet. The cold circumstellar envelope
surrounding the object has been mapped through its 1.2 mm dust continuum
emission with IRAM-30m/MAMBO. The deeply embedded protostar is clearly detected
with Spitzer/MIPS (70 um), IRS (20-35 um) and IRAC (4.5, 5.8, and 8 um) but
also in the K_s band (2.15 um). Given the large "near- and mid-IR excess" in
its spectral energy distribution, but large submillimeter-to-bolometric
luminosity ratio (~2%), IRAS 22129+7000 must be a transition Class 0/I source
and/or a multiple stellar system. Targeted observations of several molecular
lines from CO, 13CO, C18O, HCO+ and DCO+ have been obtained. The presence of a
collimated molecular outflow mapped with the CSO telescope in the CO J=3-2 line
suggests that the protostar/disk system is still accreting material from its
natal envelope. Indeed, optically thick line profiles from high density tracers
such as HCO+ J=1-0 show a red-shifted-absorption asymmetry reminiscent of
inward motions. We construct a preliminary physical model of the circumstellar
envelope (including radial density and temperature gradients, velocity field
and turbulence) that reproduces the observed line profiles and estimates the
ionization fraction. The presence of both mechanical and (non-ionizing)
FUV-radiative input makes the region an interesting case to study triggered
star formation
The Mass Distributions of Starless and Protostellar Cores in Gould Belt Clouds
Using data from the SCUBA Legacy Catalogue (850 um) and Spitzer Space
Telescope (3.6 - 70 um), we explore dense cores in the Ophiuchus, Taurus,
Perseus, Serpens, and Orion molecular clouds. We develop a new method to
discriminate submillimeter cores found by SCUBA as starless or protostellar,
using point source photometry from Spitzer wide field surveys. First, we
identify infrared sources with red colors associated with embedded young
stellar objects (YSOs). Second, we compare the positions of these
YSO-candidates to our submillimeter cores. With these identifications, we
construct new, self-consistent starless and protostellar core mass functions
(CMFs) for the five clouds. We find best fit slopes to the high-mass end of the
CMFs of -1.26 +/- 0.20, -1.22 +/- 0.06, -0.95 +/- 0.20, and -1.67 +/- 0.72 for
Ophiuchus, Taurus, Perseus, and Orion, respectively. Broadly, these slopes are
each consistent with the -1.35 power-law slope of the Salpeter IMF at higher
masses, but suggest some differences. We examine a variety of trends between
these CMF shapes and their parent cloud properties, potentially finding a
correlation between the high-mass slope and core temperature. We also find a
trend between core mass and effective size, but we are very limited by
sensitivity. We make similar comparisons between core mass and size with visual
extinction (for A_V >= 3) and find no obvious trends. We also predict the
numbers and mass distributions of cores that future surveys with SCUBA-2 may
detect in each of these clouds.Comment: 56 pages, 18 figures, fixed typo in Eq 1, results in paper remain
unchange
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