610 research outputs found
Connection between jets, winds and accretion in T Tauri stars: the X-shooter view
We have analysed the [OI]6300 A line in a sample of 131 young stars with
discs in the Lupus, Chamaeleon and signa Orionis star forming regions, observed
with the X-shooter spectrograph at VLT. The stars have mass accretion rates
spanning from 10^{-12} to 10^{-7} Mo/yr. The line profile was deconvolved into
a low velocity component (LVC,
40 km/s ), originating from slow winds and high velocity jets, respectively.
The LVC is by far the most frequent component, with a detection rate of 77%,
while only 30% of sources have a HVC. The [OI]6300 luminosity of both the LVC
and HVC, when detected, correlates with stellar and accretion parameters of the
central sources (i.e. Lstar , Mstar , Lacc , Macc), with similar slopes for the
two components. The line luminosity correlates better with the accretion
luminosity than with the stellar luminosity or stellar mass. We suggest that
accretion is the main drivers for the line excitation and that MHD disc-winds
are at the origin of both components. In the sub-sample of Lupus sources
observed with ALMA a relationship is found between the HVC peak velocity and
the outer disc inclination angle, as expected if the HVC traces jets ejected
perpendicularly to the disc plane. Mass loss rates measured from the HVC span
from ~ 10^{-13} to ~10^{-7} Mo/yr. The corresponding Mloss/Macc ratio ranges
from ~0.01 to ~0.5, with an average value of 0.07. However, considering the
upper limits on the HVC, we infer a ratio < 0.03 in more than 40% of sources.
We argue that most of these sources might lack the physical conditions needed
for an efficient magneto-centrifugal acceleration in the star-disc interaction
region. Systematic observations of populations of younger stars, that is, class
0/I, are needed to explore how the frequency and role of jets evolve during the
pre-main sequence phase.Comment: 15 pages, 14 figures, Accepted for publication in A&
Cold Disks: Spitzer Spectroscopy of Disks around Young Stars with Large Gaps
We have identified four circumstellar disks with a deficit of dust emission
from their inner 15-50 AU. All four stars have F-G spectral type, and were
uncovered as part of the Spitzer Space Telescope ``Cores to Disks'' Legacy
Program Infrared Spectrograph (IRS) first look survey of ~100 pre-main sequence
stars. Modeling of the spectral energy distributions indicates a reduction in
dust density by factors of 100-1000 from disk radii between ~0.4 and 15-50 AU,
but with massive gas-rich disks at larger radii. This large contrast between
the inner and outer disk has led us to use the term `cold disks' to distinguish
these unusual systems. However, hot dust [0.02-0.2 Mmoon] is still present
close to the central star (R ~0.8 AU). We introduce the 30/13 micron, flux
density ratio as a new diagnostic for identifying cold disks. The mechanisms
for dust clearing over such large gaps are discussed. Though rare, cold disks
are likely in transition from an optically thick to an optically thin state,
and so offer excellent laboratories for the study of planet formation.Comment: 13 pages, 3 figures, accepted to ApJ
The ultracool dwarf DENIS-P J104814.7-395606. Chromospheres and coronae at the low-mass end of the main-sequence
We have obtained an XMM-Newton observation and a broad-band spectrum from the
ultraviolet to the near infrared with X-Shooter for one of the nearest M9
dwarfs, DENIS-P J1048-3956 (4pc). We integrate these data by a compilation of
activity parameters for ultracool dwarfs from the literature with the aim to
advance our understanding of these objects by comparing them to early-M type
dwarf stars and the Sun.
Our deep XMM-Newton observation has led to the first X-ray detection of
DENIS-P J1048-3956 (log Lx = 25.1) as well as the first measurement of its V
band brightness (V = 17.35mag). Flux-flux relations between X-ray and
chromospheric activity indicators are here for the first time extended into the
regime of the ultracool dwarfs. The approximate agreement of DENIS-P J1048-3956
and other ultracool dwarfs with flux-flux relations for early-M dwarfs suggests
that the same heating mechanisms work in the atmospheres of ultracool dwarfs,
albeit weaker as judged from their lower fluxes. The observed Balmer decrements
of DENIS-P J1048-3956 are compatible with optically thick plasma in LTE at low,
nearly photospheric temperature or optically thin LTE plasma at 20000K.
Describing the decrements with CaseB recombination requires different emitting
regions for Halpha and the higher Balmer lines. The high observed Halpha/Hbeta
flux ratio is also poorly fitted by the optically thin models. We derive a
similarly high value for the Halpha/Hbeta ratio of vB10 and LHS2065 and
conclude that this may be a characteristic of ultracool dwarfs. We add DENIS-P
J1048-3956 to the list of ultracool dwarfs detected in both the radio and the
X-ray band. The Benz-Guedel relation between radio and X-ray luminosity of
late-type stars is well-known to be violated by ultracool dwarfs. We speculate
on the presence of two types of ultracool dwarfs with distinct radio and X-ray
behavior.Comment: accepted for publication in Astronomy & Astrophysic
A new Classical T Tauri object at the sub-stellar boundary in Chamaeleon II
We have obtained low- and medium-resolution optical spectra of 20 candidate
young low-mass stars and brown dwarfs in the nearby Chamaeleon II dark cloud,
using the Magellan Baade telescope. We analyze these data in conjunction with
near-infrared photometry from the 2-Micron All Sky Survey. We find that one
target, [VCE2001] C41, exhibits broad H(alpha) emission as well as a variety of
forbidden emission lines. These signatures are usually associated with
accretion and outflow in young stars and brown dwarfs. Our spectra of C41 also
reveal LiI in absorption and allow us to derive a spectral type of M5.5 for it.
Therefore, we propose that C41 is a classical T Tauri object near the
sub-stellar boundary. Thirteen other targets in our sample have continuum
spectra without intrinsic absorption or emission features, and are difficult to
characterize. They may be background giants or foreground field stars not
associated with the cloud or embedded protostars, and need further
investigation. The six remaining candidates, with moderate reddening, are
likely to be older field dwarfs, given their spectral types, lack of lithium
and H(alpha).Comment: Astrophysical Journal, accepted June 19, 200
X-Shooter spectroscopy of young stellar objects in Lupus: Accretion properties of class II and transitional objects
We present the results of a study of the stellar and accretion properties of
the (almost) complete sample of class II and transitional YSOs in the Lupus I,
II, III and IV clouds, based on spectroscopic data acquired with the
VLT/X-Shooter spectrograph. Our study combines the dataset from our previous
work with new observations of 55 additional objects. We have investigated 92
YSO candidates in total, 11 of which have been definitely identified with giant
stars unrelated to Lupus. The stellar and accretion properties of the 81 bona
fide YSOs, which represent more than 90% of the whole class~II and transition
disc YSO population in the aforementioned Lupus clouds, have been homogeneously
and self-consistently derived, allowing for an unbiased study of accretion and
its relationship with stellar parameters. The accretion luminosity, Lacc,
increases with the stellar luminosity, Lstar, with an overall slope of ~1.6,
similar but with a smaller scatter than in previous studies. There is a
significant lack of strong accretors below Lstar~0.1Lsun, where Lacc is always
lower than 0.01Lstar. We argue that the Lacc-Lstar slope is not due to
observational biases, but is a true property of the Lupus YSOs. The
logMacc-logMstar correlation shows a statistically significant evidence of a
break, with a steeper relation for Mstar<0.2Msun and a flatter slope for higher
masses. The bimodality of the Macc-Mstar relation is confirmed with four
different evolutionary models used to derive the stellar mass. The bimodal
behaviour of the observed relationship supports the importance of modelling
self-gravity in the early evolution of the more massive discs, but other
processes, such as photo evaporation and planet formation during the YSO's
lifetime, may also lead to disc dispersal on different timescales depending on
the stellar mass. We also refined the empirical Lacc vs. Lline relationships.Comment: 43 pages, 22 figure
Intracluster stars in the Virgo cluster core
We have investigated the properties of the diffuse light in the Virgo cluster
core region, based on the detection of intracluster planetary nebulae (PNe) in
four fields. We eliminate the bias from misclassified faint continuum objects,
using improved Monte Carlo simulations, and the contaminations by high redshift
Ly galaxies, using the Ly luminosity function in blank fields.
Recent spectroscopic observations confirm that our photometric PN samples are
well-understood. We find that the diffuse stellar population in the Virgo core
region is inhomogeneous on scales of 30'-90': there exist significant
field-to-field variations in the number density of PNe and the inferred amount
of intracluster light, with some empty fields, some fields dominated by
extended Virgo galaxy halos, and some fields dominated by the true intracluster
component. There is no clear trend with distance from M87. The mean surface
luminosity density, its rms variation, and the mean surface brightness of
diffuse light in our 4 fields are L
arcmin, L arcmin, and
mag arcsec respectively. Our results indicate that
the Virgo cluster is a dynamically young environment, and that the intracluster
component is associated at least partially with local physical processes like
galaxy interactions or harassment. We also argue, based on kinematic evidence,
that the so-called 'over-luminous' PNe in the halo of M84 are dynamically
associated with this galaxy, and must thus be brighter than and part of a
different stellar population from the normal PN population in elliptical
galaxies.Comment: 31 pages, 6 figure. In press on the Astronomical Journa
The role of marine reserves in achieving sustainable fisheries (One contribution of 15 to a Theme Issue 'Fisheries: a Future?')
Many fishery management tools currently in use have conservation value. They are designed to maintain stocks of commercially important species above target levels. However, their limitations are evident from continuing declines in fish stocks throughout the world. We make the case that to reverse fishery declines, safeguard marine life and sustain ecosystem processes, extensive marine reserves that are off limits to fishing must become part of the management strategy. Marine reserves should be incorporated into modern fishery management because they can achieve many things that conventional tools cannot. Only complete and permanent protection from fishing can protect the most sensitive habitats and vulnerable species. Only reserves will allow the development of natural, extended age structures of target species, maintain their genetic variability and prevent deleterious evolutionary change from the effects of fishing. Species with natural age structures will sustain higher rates of reproduction and will be more resilient to environmental variability. Higher stock levels maintained by reserves will provide insurance against management failure, including risk-prone quota setting, provided the broader conservation role of reserves is firmly established and legislatively protected. Fishery management measures outside protected areas are necessary to complement the protection offered by marine reserves, but cannot substitute for it
X-rays and Protostars in the Trifid Nebula
The Trifid Nebula is a young HII region recently rediscovered as a
"pre-Orion" star forming region, containing protostars undergoing violent mass
ejections visible in optical jets as seen in images from the Infrared Space
Observatory and the Hubble Space Telescope. We report the first X-ray
observations of the Trifid nebula using ROSAT and ASCA. The ROSAT image shows a
dozen X-ray sources, with the brightest X-ray source being the O7 star, HD
164492, which provides most of the ionization in the nebula. We also identify
85 T Tauri star and young, massive star candidates from near-infrared colors
using the JHKs color-color diagram from the Two Micron All Sky Survey (2MASS).
Ten X-ray sources have counterpart near-infrared sources. The 2MASS stars and
X-ray sources suggest there are potentially numerous protostars in the young
HII region of the Trifid. ASCA moderate resolution spectroscopy of the
brightest source shows hard emission up to 10 keV with a clearly detected Fe K
line. The best model fit is a two-temperature (T = 1.2x10^6 K and 39x10^6 K)
thermal model with additional warm absorbing media. The hotter component has an
unusually high temperature for either an O star or an HII region; a typical
Galactic HII region could not be the primary source for such hot temperature
plasma and the Fe XXV line emission. We suggest that the hotter component
originates in either the interaction of the wind with another object (a
companion star or a dense region of the nebula) or from flares from deeply
embedded young stars.Comment: Accepted in ApJ (Oct, 20 issue, 2001
The Stellar Population of the Chamaeleon I Star-Forming Region
I present a new census of the stellar population in the Chamaeleon I
star-forming region. Using optical and near-IR photometry and followup
spectroscopy, I have discovered 50 new members of Chamaeleon I, expanding the
census of known members to 226 objects. Fourteen of these new members have
spectral types later than M6, which doubles the number of known members that
are likely to be substellar. I have estimated extinctions, luminosities, and
effective temperatures for the known members, used these data to construct an
H-R diagram for the cluster, and inferred individual masses and ages with the
theoretical evolutionary models of Baraffe and Chabrier. The distribution of
isochronal ages indicates that star formation began 3-4 and 5-6 Myr ago in the
southern and northern subclusters, respectively, and has continued to the
present time at a declining rate. The IMF in Chamaeleon I reaches a maximum at
a mass of 0.1-0.15 M_sun, and thus closely resembles the IMFs in IC 348 and the
Orion Nebula Cluster. In logarithmic units where the Salpeter slope is 1.35,
the IMF is roughly flat in the substellar regime and shows no indication of
reaching a minimum down to a completeness limit of 0.01 M_sun. The low-mass
stars are more widely distributed than members at other masses in the northern
subcluster, but this is not the case in the southern subcluster. Meanwhile, the
brown dwarfs have the same spatial distribution as the stars out to a radius of
3 deg (8.5 pc) from the center of Chamaeleon I
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