262 research outputs found
Linear spectro-polarimetry: a new diagnostic tool for the classification and characterisation of asteroids
We explore the use of spectro-polarimetry as a remote sensing tool for
asteroids in addition to traditional reflectance measurements. In particular we
are interested in possible relationships between the wavelength-dependent
variation of linear polarization and the properties of the surfaces, including
albedo and composition.
We have obtained optical spectro-polarimetric measurements of a dozen
asteroids of different albedo and taxonomic classes and of two small regions at
the limb of the Moon.
We found that objects with marginally different relative reflectance spectra
(in the optical) may have totally different polarization spectra. This suggests
that spectro-polarimetry may be used to refine the classification of asteroids.
We also found that in some cases the Umov law may be violated, that is, in
contrast to what is expected from basic physical considerations, the fraction
of linear polarization and the reflectance may be positively correlated. In
agreement with a few previous studies based on multi-colour broadband
polarimetry, we found that the variation of linear polarization with wavelength
and with phase-angle is correlated with the albedo and taxonomic class of the
objects. Finally, we have serendipitously discovered that spinel-rich asteroid
(599) Luisa, located very close to the Watsonia family, is a member of the rare
class of Barbarian asteroids.
We suggest that future modelling attempts of the surface structure of
asteroids should be aimed at explaining both reflectance and polarization
spectra.Comment: Accepted for publication in MNRAS Letter
Search for associations containing young stars (SACY). V. Is multiplicity universal? Tight multiple systems
Context: Dynamically undisrupted, young populations of stars are crucial to
study the role of multiplicity in relation to star formation. Loose nearby
associations provide us with a great sample of close (150 pc) Pre-Main
Sequence (PMS) stars across the very important age range (5-70 Myr) to
conduct such research.
Aims: We characterize the short period multiplicity fraction of the SACY
(Search for Associations Containing Young stars) accounting for any
identifiable bias in our techniques and present the role of multiplicity
fractions of the SACY sample in the context of star formation.
Methods: Using the cross-correlation technique we identified double-lined
spectroscopic systems (SB2), in addition to this we computed Radial Velocity
(RV) values for our subsample of SACY targets using several epochs of FEROS and
UVES data. These values were used to revise the membership of each association
then combined with archival data to determine significant RV variations across
different data epochs characteristic of multiplicity; single-lined multiple
systems (SB1).
Results: We identified 7 new multiple systems (SB1s: 5, SB2s: 2). We find no
significant difference between the short period multiplicity fraction
() of the SACY sample and that of nearby star forming regions
(1-2 Myr) and the field (10%) both as a function of
age and as a function of primary mass, , in the ranges [1:200 day] and
[0.08 -].
Conclusions: Our results are consistent with the picture of universal star
formation, when compared to the field and nearby star forming regions (SFRs).
We comment on the implications of the relationship between increasing
multiplicity fraction with primary mass, within the close companion range, in
relation to star formation.Comment: 14 pages, 18 figures, published, A&A
http://dx.doi.org/10.1051/0004-6361/20142385
Comparative statistics and origin of triple and quadruple stars
The statistics of catalogued quadruple stars consisting of two binaries
(hierarchy 2+2) is studied in comparison with triple stars, with respective
sample sizes of 81 and 724. Seven representative quadruple systems are
discussed in greater detail. The properties of multiple stars do not correspond
to the products of dynamical decay of small clusters, hence the N-body dynamics
is not the dominant process of their formation. On the other hand,
rotationally-driven (cascade) fragmentation possibly followed by migration of
inner and/or outer orbits to shorter periods is a promising scenario to explain
the origin of triple and quadruple stars. Our main results are: (i) Quadruple
systems of Epsilon Lyr type with similar masses and inner periods are common.
(ii) The distributions of the inner periods in triple and quadruple stars are
similar and bimodal. The inner mass ratios do not correlate with the inner
periods. (iii) The statistics of outer periods and mass ratios in triples and
quadruples are different. The median outer mass ratio in triples is 0.39
independently of the outer period, which has a smooth distribution. In
contrast, the outer periods of 25% quadruples concentrate in the narrow range
from 10yr to 100yr, the outer mass ratios of these tight quadruples are above
0.6 and their two inner periods are similar to each other. (iv) The outer and
inner mass ratios in triple and quadruple stars are not mutually correlated.
(v) The inner and outer orbital angular momenta and periods in triple and
quadruple systems with inner periods above 30d show some correlation, the ratio
of outer-to-inner periods is mostly comprised between 5 and 10^4. In the
systems with small period ratios the directions of the orbital spins are
correlated, while in the systems with large ratios they are not.Comment: Accepted by MNRAS, 14 pages, 12 figures. Two electronic tables at
http://www.ctio.noao.edu/ftp/pub/tokovinin/quadruples
Search for Associations Containing Young stars (SACY): Chemical tagging IC 2391 & the Argus association
We explore the possible connection between the open cluster IC 2391 and the
unbound Argus association identified by the SACY survey. In addition to common
kinematics and ages between these two systems, here we explore their chemical
abundance patterns to confirm if the two substructures shared a common origin.
We carry out a homogenous high-resolution elemental abundance study of eight
confirmed members of IC 2391 as well as six members of the Argus association
using UVES spectra. We derive spectroscopic stellar parameters and abundances
for Fe, Na, Mg, Al, Si, Ca, Ti, Cr, Ni and Ba.
All stars in the open cluster and Argus association were found to share
similar abundances with the scatter well within the uncertainties, where [Fe/H]
= -0.04 +/-0.03 for cluster stars and [Fe/H] = -0.06 +/-0.05 for Argus stars.
Effects of over-ionisation/excitation were seen for stars cooler than roughly
5200K as previously noted in the literature. Also, enhanced Ba abundances of
around 0.6 dex were observed in both systems. The common ages, kinematics and
chemical abundances strongly support that the Argus association stars
originated from the open cluster IC 2391. Simple modeling of this system find
this dissolution to be consistent with two-body interactions.Comment: 17 pages, 7 figs, accepted for publication in MNRA
Search for associations containing young stars (SACY) VII. New stellar and substellar candidate members in the young associations
The young associations offer us one of the best opportunities to study the
properties of young stellar and substellar objects and to directly image
planets thanks to their proximity (200 pc) and age (5-150 Myr).
However, many previous works have been limited to identifying the brighter,
more active members (1 M) owing to photometric survey
sensitivities limiting the detections of lower mass objects. We search the
field of view of 542 previously identified members of the young associations to
identify wide or extremely wide (1000-100,000 au in physical separation)
companions. We combined 2MASS near-infrared photometry (, , ) with
proper motion values (from UCAC4, PPMXL, NOMAD) to identify companions in the
field of view of known members. We collated further photometry and spectroscopy
from the literature and conducted our own high-resolution spectroscopic
observations for a subsample of candidate members. This complementary
information allowed us to assess the efficiency of our method. We identified 84
targets (45: 0.2-1.3 M, 17: 0.08-0.2 M, 22: 0.08 M)
in our analysis, ten of which have been identified from spectroscopic analysis
in previous young association works. For 33 of these 84, we were able to
further assess their membership using a variety of properties (X-ray emission,
UV excess, H, lithium and K I equivalent widths, radial velocities,
and CaH indices). We derive a success rate of 76-88% for this technique based
on the consistency of these properties. Once confirmed, the targets identified
in this work would significantly improve our knowledge of the lower mass end of
the young associations. Additionally, these targets would make an ideal new
sample for the identification and study of planets around nearby young stars.Comment: 28 pages, 24 figures, accepted in A&
A spectroscopic survey of the youngest field stars in the solar neighbourhood. I. The optically bright sample
We present the first results of a ground-based programme conducted on 1-4m
class telescopes. Our sample consists of 1097 active and presumably young
stars, all of them being optical counterparts of RASS X-ray sources in the
northern hemisphere. We concentrate on the 704 optically brightest
(V_Ticho<=9.5 mag) candidates. We acquired high-res spectroscopy in the
Halpha/Li spectral regions for 426 of such stars without relevant literature
data. We describe the sample and the observations and we start to discuss its
physical properties. We used a cross-correlation technique and other tools to
derive accurate radial/rotational velocities and to perform a spectral
classification for both single and SB2 stars. The spectral subtraction
technique was used to derive chromospheric activity levels and Li abundances.
We estimated the fraction of young single stars and multiple systems in stellar
soft X-ray surveys and the contamination by more evolved systems, like RS
CVn's. We classified stars on the basis of Li abundance and give a glimpse of
their sky distribution. The sample appears to be a mixture of young
Pleiades-/Hyades- like stars plus an older Li-poor population (~1-2 Gyr). 7
stars with Li abundance compatible with the age of IC 2602 (~30 Myr) or younger
were detected as well, although 2 appear to be Li-rich giants. The discovery of
a large number of Li-rich giants is another outcome of this survey. The
contamination of soft X-ray surveys by old systems in which the activity level
is enhanced by tidal synchronisation is not negligible, especially for K-type
stars. 5 stars with Li content close to the primordial abundance are probably
associated with known moving groups in the solar neighbourhood. Some of them
are PTTS candidates according to their positions in the HR diagram.Comment: 16 pages, 12 figures, 6 tables; 2 figures and 2 tables in electronic
form only. Paper accepted by Astronomy and Astrophysic
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
Discovery of "isolated" comoving T Tauri stars in Cepheus
During the course of a large spectroscopic survey of X-ray active late-type
stars in the solar neighbourhood, we discovered four lithium-rich stars packed
within just a few degrees on the sky. These very young stars are projected
several degrees away from the Cepheus-Cassiopea clouds, in front of an area
void of interstellar matter. As such, they are very good "isolated" T Tauri
star candidates. We acquired high-resolution optical spectra as well as
photometric data allowing us to investigate in detail their nature and physical
parameters with the aim of testing the "runaway" and "in-situ" formation
scenarios. We derive accurate radial and rotational velocities and perform an
automatic spectral classification. The spectral subtraction technique is used
to infer chromospheric activity level in the H-alpha line core and clean the
spectra of photospheric lines before measuring the equivalent width of the
lithium absorption line. Both physical (lithium content, magnetic activity) and
kinematical indicators show that all stars are very young (ages in the range
10-30 Myr). In particular, the spectral energy distribution of TYC4496-780-1
displays a strong near- and far-infrared excess, typical of T Tauri stars still
surrounded by an accretion disc. They also share the same Galactic motion,
proving that they form a homogeneous moving group of stars with the same
origin. The most plausible explanation of how these "isolated" T Tauri stars
formed is the "in-situ" model, although accurate distances are needed to
clarify their connection with the Cepheus-Cassiopeia complex
- …