832 research outputs found
A Possible Massive Asteroid Belt Around zeta Lep
We have used the Keck I telescope to image at 11.7 microns and 17.9 microns
the dust emission around zeta Lep, a main sequence A-type star at 21.5 pc from
the Sun with an infrared excess. The excess is at most marginally resolved at
17.9 microns. The dust distance from the star is probably less than or equal to
6 AU, although some dust may extend to 9 AU. The mass of observed dust is
\~10^22 g. Since the lifetime of dust particles is about 10,000 years because
of the Poytning-Robertson effect, we robustly estimate at least 4 10^26 g must
reside in parent bodies which may be asteroids if the system is in a steady
state and has an age of ~300 Myr. This mass is approximately 200 times that
contained within the main asteroid belt in our solar system.Comment: 12 pages, 3 figures, ApJL in pres
FLAMES spectroscopy of low-mass stars in the young clusters sigma Ori and lambda Ori
Aims. We performed a detailed membership selection and studied the accretion
properties of low-mass stars in the two apparently very similar young (1-10
Myr) clusters sigma Ori and lambda Ori.
Methods. We observed 98 and 49 low-mass (0.2-1.0 M_sun) stars in sigma Ori
and lambda Ori respectively, using the multi-object optical spectrograph FLAMES
at the VLT, with the high-resolution (R=17,000) HR15N grating (6470-6790 AA).
We used radial velocities, Li and Halpha to establish cluster membership and
Halpha and other optical emission lines to analyze the accretion properties of
members.
Results. We identified 65 and 45 members of the sigma Ori and lambda Ori
clusters, respectively and discovered 16 new candidate binary systems. We also
measured rotational broadening for 20 stars and estimated the mass accretion
rates in 25 stars of the sigma Ori cluster, finding values between 10^-11 and
10^-7.7 M_sun yr^-1 and in 4 stars of the lambda Ori cluster, finding values
between 10^-11 and 10^-10.1 M_sun yr-1. Comparing our results with the infrared
photometry obtained by the Spitzer satellite, we find that the fraction of
stars with disks and the fraction of active disks is larger in the sigma Ori
cluster (52+-9% and 78+-16%) than in lambda Ori (28+-8% and 40+-20%)
Conclusions. The different disk and accretion properties of the two clusters
could be due either to the effect of the high-mass stars and the supernova
explosion in the lambda Ori cluster or to different ages of the cluster
populations. Further observations are required to draw a definitive conclusion.Comment: 14 pages, 9 figures, 9 tables, accepted for publications in A&
Search for associations containing young stars (SACY). VI. Is multiplicity universal? Stellar multiplicity in the range 3-1000 au from adaptive-optics observations
Context. Young loose nearby associations are unique samples of close (<150
pc), young (approx 5-100 Myr) pre-main sequence (PMS) stars. A significant
number of members of these associations have been identified in the SACY
collaboration. We can use the proximity and youth of these members to
investigate key ingredients in star formation processes, such as multiplicity.
Aims. We present the statistics of identified multiple systems from 113
confirmed SACY members. We derive multiplicity frequencies, mass-ratio, and
physical separation distributions in a consistent parameter space, and compare
our results to other PMS populations and the field.
Methods. We have obtained adaptive-optics assisted near-infrared observations
with NACO (ESO/VLT) and IRCAL (Lick Observatory) for at least one epoch of all
113 SACY members. We have identified multiple systems using co-moving
proper-motion analysis and using contamination estimates. We have explored
ranges in projected separation and mass-ratio of a [3-1000 au], and q [0.1-1],
respectively.
Results. We have identified 31 multiple systems (28 binaries and 3 triples).
We derive a multiplicity frequency (MF) of MF_(3-1000au)=28.4 +4.7, -3.9% and a
triple frequency (TF) of TF_(3-1000au)=2.8 +2.5, -0.8% in the separation range
of 3-1000 au. We do not find any evidence for an increase in the MF with
primary mass. The estimated mass-ratio of our statistical sample (with
power-law index gamma=-0.04 +/- 0.14) is consistent with a flat distribution
(gamma = 0).
Conclusions. We show further similarities (but also hints of discrepancies)
between SACY and the Taurus region: flat mass-ratio distributions and
statistically similar MF and TF values. We also compared the SACY sample to the
field (in the separation range of 19-100 au), finding that the two
distributions are indistinguishable, suggesting a similar formation mechanism.Comment: 16 Pages, accepted in A&A 28 May 201
The Lithium Depletion Boundary and the Age of the Young Open Cluster IC~2391
We have obtained new photometry and intermediate resolution ( \AA\ ) spectra of 19 of these objects
(14.9 17.5) in order to confirm cluster membership. We
identify 15 of our targets as likely cluster members based on their
photometry, spectral types, radial velocity, and H emission strengths.
Higher S/N spectra were obtained for 8 of these probable cluster members in
order to measure the strength of the lithium 6708 \AA\ doublet and thus obtain
an estimate of the cluster's age. One of these 8 stars has a definite lithium
detection and two other (fainter) stars have possible lithium detections. A
color-magnitude diagram for our program objects shows that the lithium
depletion boundary in IC~2391 is at =16.2. Using recent theoretical model
predictions, we derive an age for IC~2391 of 535 Myr. While this is
considerably older than the age most commonly attributed for this cluster
(35 Myr) this result for IC~2391 is comparable those recently derived for
the Pleiades and Alpha Persei clusters and can be explained by new models for
high mass stars that incorporate a modest amount of convective core
overshooting.Comment: ApJ Letters, acccepte
The Highly Eccentric Pre-Main Sequence Spectroscopic Binary RX J0529.3+1210
The young system RX J0529.3+1210 was initially identified as a single-lined
spectroscopic binary. Using high-resolution infrared spectra, acquired with
NIRSPEC on Keck II, we measured radial velocities for the secondary. The method
of using the infrared regime to convert single-lined spectra into double-lined
spectra, and derive the mass ratio for the binary system, has been successfully
used for a number of young, low-mass binaries. For RX J0529.3+1210, a long-
period(462 days) and highly eccentric(0.88) binary system, we determine the
mass ratio to be 0.78+/-0.05 using the infrared double-lined velocity data
alone, and 0.73+/-0.23 combining visible light and infrared data in a full
orbital solution. The large uncertainty in the latter is the result of the
sparse sampling in the infrared and the high eccentricity: the stars do not
have a large velocity separation during most of their ~1.3 year orbit. A mass
ratio close to unity, consistent with the high end of the one sigma uncertainty
for this mass ratio value, is inconsistent with the lack of a visible light
detection of the secondary component. We outline several scenarios for a color
difference in the two stars, such as one heavily spotted component, higher
order multiplicity, or a unique evolutionary stage, favoring detection of only
the primary star in visible light, even in a mass ratio ~1 system. However, the
evidence points to a lower ratio. Although RX J0529.3+1210 exhibits no excess
at near-infrared wavelengths, a small 24 micron excess is detected, consistent
with circumbinary dust. The properties of this binary and its membership in
Lambda Ori versus a new nearby stellar moving group at ~90 pc are discussed. We
speculate on the origin of this unusual system and on the impact of such high
eccentricity on the potential for planet formation.Comment: 4 Figure
Combinatorial Intracellular Delivery Screening of Anticancer Drugs
Conventional drug solubilization strategies limit the understanding of the full potential of poorly water-soluble drugs during drug screening. Here, we propose a screening approach in which poorly water-soluble drugs are entrapped in poly(2-(methacryloyloxyethyl phosphorylcholine)-poly(2-(diisopropylaminoethyl methacryate) (PMPC–PDPA) polymersomes (POs) to enhance drug solubility and facilitate intracellular delivery. By using a human pediatric glioma cell model, we demonstrated that PMPC–PDPA POs mediated intracellular delivery of cytotoxic and epigenetic drugs by receptor-mediated endocytosis. Additionally, when delivered in combination, drug-loaded PMPC–PDPA POs triggered both an enhanced drug efficacy and synergy compared to that of a conventional combinatorial screening. Hence, our comprehensive synergy analysis illustrates that our screening methodology, in which PMPC–PDPA POs are used for intracellular codelivery of drugs, allows us to identify potent synergistic profiles of anticancer drugs
Pre-main-sequence Lithium Depletion
In this review I briefly discuss the theory of pre-main-sequence (PMS) Li
depletion in low-mass (0.075<M<1.2 Msun) stars and highlight those uncertain
parameters which lead to substantial differences in model predictions. I then
summarise observations of PMS stars in very young open clusters, clusters that
have just reached the ZAMS and briefly highlight recent developments in the
observation of Li in very low-mass PMS stars.Comment: 8 pages, invited review at "Chemical abundances and mixing in stars
in the Milky Way and its satellites", eds. L. Pasquini, S. Randich. ESO
Astrophysics Symposium (Springer-Verlag
The effect of stellar activity on the LiI 6708, NaI 5896 and KI 7699 \AA lines: A comparison with the Pleiades, field stars and the Sun
An analytical model has been developed to empirically study the effects of stellar spots and faculae on the observed equivalent widths of Li {\sc i} 6708, Na {\sc i} 5896 and K {\sc i} 7699 \AA lines (and abundances in the case of lithium) in late-type stars, taking into account the changes in the observed magnitudes and colors. Solar spectra corresponding to different active regions are used as input data and a range of filling factors are applied to simulate the surfaces of stars with different levels of activity. Detailed comparisons between predicted and observed photometric colors and equivalent widths are made for late-type stars of the Pleiades and the field. The observed dispersions in K {\sc i} and Li {\sc i} equivalent widths for Pleiades stars can be partially accounted by the simultaneous effects of activity on colors and the line formation, indicating that the lithium-rotation connection suggested for ~0.7-0.9 Msun Pleiades stars could be due in part to the stellar activity. However, under realistic values for the filling factors, only a small portion of the observed spread could be explained by these effect
Gas and dust in the Beta Pictoris Moving Group as seen by the Herschel Space Observatory
Context. Debris discs are thought to be formed through the collisional
grinding of planetesimals, and can be considered as the outcome of planet
formation. Understanding the properties of gas and dust in debris discs can
help us to comprehend the architecture of extrasolar planetary systems.
Herschel Space Observatory far-infrared (IR) photometry and spectroscopy have
provided a valuable dataset for the study of debris discs gas and dust
composition. This paper is part of a series of papers devoted to the study of
Herschel PACS observations of young stellar associations.
Aims. This work aims at studying the properties of discs in the Beta Pictoris
Moving Group (BPMG) through far-IR PACS observations of dust and gas.
Methods. We obtained Herschel-PACS far-IR photometric observations at 70, 100
and 160 microns of 19 BPMG members, together with spectroscopic observations of
four of them. Spectroscopic observations were centred at 63.18 microns and 157
microns, aiming to detect [OI] and [CII] emission. We incorporated the new
far-IR observations in the SED of BPMG members and fitted modified blackbody
models to better characterise the dust content.
Results. We have detected far-IR excess emission toward nine BPMG members,
including the first detection of an IR excess toward HD 29391.The star HD
172555, shows [OI] emission, while HD 181296, shows [CII] emission, expanding
the short list of debris discs with a gas detection. No debris disc in BPMG is
detected in both [OI] and [CII]. The discs show dust temperatures in the range
55 to 264 K, with low dust masses (6.6*10^{-5} MEarth to 0.2 MEarth) and radii
from blackbody models in the range 3 to 82 AU. All the objects with a gas
detection are early spectral type stars with a hot dust component.Comment: 12 pages, 7 figures, 6 table
Panchromatic observations and modeling of the HV Tau C edge-on disk
We present new high spatial resolution (<~ 0.1") 1-5 micron adaptive optics
images, interferometric 1.3 mm continuum and 12CO 2-1 maps, and 350 micron, 2.8
and 3.3 mm fluxes measurements of the HV Tau system. Our adaptive optics images
reveal an unusually slow orbital motion within the tight HV Tau AB pair that
suggests a highly eccentric orbit and/or a large deprojected physical
separation. Scattered light images of the HV Tau C edge-on protoplanetary disk
suggest that the anisotropy of the dust scattering phase function is almost
independent of wavelength from 0.8 to 5 micron, whereas the dust opacity
decreases significantly over the same range. The images further reveal a marked
lateral asymmetry in the disk that does not vary over a timescale of 2 years.
We further detect a radial velocity gradient in the disk in our 12CO map that
lies along the same position angle as the elongation of the continuum emission,
which is consistent with Keplerian rotation around an 0.5-1 Msun central star,
suggesting that it could be the most massive component in the triple system. We
use a powerful radiative transfer model to compute synthetic disk observations
and use a Bayesian inference method to extract constraints on the disk
properties. Each individual image, as well as the spectral energy distribution,
of HV Tau C can be well reproduced by our models with fully mixed dust provided
grain growth has already produced larger-than-interstellar dust grains.
However, no single model can satisfactorily simultaneously account for all
observations. We suggest that future attempts to model this source include more
complex dust properties and possibly vertical stratification. (Abridged)Comment: 26 pages, 11 figures, editorially accepted for publication in Ap
- …