897 research outputs found
X-Shooter study of accretion in -Ophiucus: very low-mass stars and brown dwarfs
We present new VLT/X-Shooter optical and NIR spectra of a sample of 17
candidate young low-mass stars and BDs in the rho-Ophiucus cluster. We derived
SpT and Av for all the targets, and then we determined their physical
parameters. All the objects but one have M*<0.6 Msun, and 8 have mass below or
close to the hydrogen-burning limit. Using the intensity of various emission
lines present in their spectra, we determined the Lacc and Macc for all the
objects. When compared with previous works targeting the same sample, we find
that, in general, these objects are not as strongly accreting as previously
reported, and we suggest that the reason is our more accurate estimate of the
photospheric parameters. We also compare our findings with recent works in
other slightly older star-forming regions to investigate possible differences
in the accretion properties, but we find that the accretion properties for our
targets have the same dependence on the stellar and substellar parameters as in
the other regions. This leads us to conclude that we do not find evidence for a
different dependence of Macc with M* when comparing low-mass stars and BDs.
Moreover, we find a similar small (1 dex) scatter in the Macc-M* relation as in
some of our recent works in other star-forming regions, and no significant
differences in Macc due to different ages or properties of the regions. The
latter result suffers, however, from low statistics and sample selection biases
in the current studies. The small scatter in the Macc-M* correlation confirms
that Macc in the literature based on uncertain photospheric parameters and
single accretion indicators, such as the Ha width, can lead to a scatter that
is unphysically large. Our studies show that only broadband spectroscopic
surveys coupled with a detailed analysis of the photospheric and accretion
properties allows us to properly study the evolution of disk accretion rates.Comment: accepted for publication in Astronomy & Astrophysics. Abstract
shortened to fit arXiv constraint
Contemporaneous broad-band photometry and H observations of T Tauri stars
The study of contemporaneous variations of the continuum flux and emission
lines is of great importance to understand the different astrophysical
processes at work in T Tauri stars. In this paper we present the results of a
simultaneous and H photometric monitoring, contemporaneous to
medium-resolution spectroscopy of six T Tauri stars in the Taurus-Auriga star
forming region. We have characterized the H photometric system using
synthetic templates and the contemporaneous spectra of the targets. We show
that we can achieve a precision corresponding to 23 \AA\ in the H
equivalent width, in typical observing conditions. The spectral analysis has
allowed us to determine the basic stellar parameters and the values of
quantities related to the accretion. In particular, we have measured a
significant veiling only for the three targets with the strongest H
emission (T Tau, FM Tau, and DG Tau). The broad-band photometric variations are
found to be in the range 0.050.70 mag and are often paired to variations in
the H intensity, which becomes stronger when the stellar continuum is
weaker. In addition, we have mostly observed a redder and a bluer
color as the stars become fainter. For most of the targets, the timescales of
these variations seem to be longer than the rotation period. One exception is T
Tau, for which the broad-band photometry varies with the rotation period. The
most plausible interpretation of these photometric and H variations is
that they are due to non-stationary mass accretion onto the stars, but
rotational modulation can play a major role in some cases.Comment: 21 pages, 11 figures, accepted for publication in Acta Astronomic
Elemental abundances of low-mass stars in nearby young associations: AB Doradus, Carina Near, and Ursa Major
We present stellar parameters and abundances of 11 elements (Li, Na, Mg, Al,
Si, Ca, Ti, Cr, Fe, Ni, and Zn) of 13 F6-K2 main-sequence stars in the young
groups AB Doradus, Carina Near, and Ursa Major. The exoplanet-host star \iota
Horologii is also analysed.
The three young associations have lithium abundance consistent with their
age. All other elements show solar abundances. The three groups are
characterised by a small scatter in all abundances, with mean [Fe/H] values of
0.10 (\sigma=0.03), 0.08 (\sigma=0.05), and 0.01 (\sigma=0.03) dex for AB
Doradus, Carina Near, and Ursa Major, respectively. The distribution of
elemental abundances appears congruent with the chemical pattern of the
Galactic thin disc in the solar vicinity, as found for other young groups. This
means that the metallicity distribution of nearby young stars, targets of
direct-imaging planet-search surveys, is different from that of old, field
solar-type stars, i.e. the typical targets of radial velocity surveys.
The young planet-host star \iota Horologii shows a lithium abundance lower
than that found for the young association members. It is found to have a
slightly super-solar iron abundance ([Fe/H]=0.16+-0.09), while all [X/Fe]
ratios are similar to the solar values. Its elemental abundances are close to
those of the Hyades cluster derived from the literature, which seems to
reinforce the idea of a possible common origin with the primordial cluster.Comment: 16 pages, 2 figures, 6 tables. Accepted for publication in MNRA
The VISTA Orion mini-survey: star formation in the Lynds 1630 North cloud
The Orion cloud complex presents a variety of star formation mechanisms and
properties and it is still one of the most intriguing targets for star
formation studies. We present VISTA/VIRCAM near-infrared observations of the
L1630N star forming region, including the stellar clusters NGC 2068 and NGC
2071, in the Orion molecular cloud B and discuss them in combination with
Spitzer data. We select 186 young stellar object (YSO) candidates in the region
on the basis of multi-colour criteria, confirm the YSO nature of the majority
of them using published spectroscopy from the literature, and use this sample
to investigate the overall star formation properties in L1630N. The K-band
luminosity function of L1630N is remarkably similar to that of the Trapezium
cluster, i.e., it presents a broad peak in the range 0.3-0.7 M and a
fraction of sub-stellar objects of 20%. The fraction of YSOs still
surrounded by disk/envelopes is very high (85%) compared to other star
forming regions of similar age (1-2 Myr), but includes some uncertain
corrections for diskless YSOs. Yet, a possibly high disk fraction together with
the fact that 1/3 of the cloud mass has a gas surface density above the
threshold for star formation (129 M pc), points towards a
still on-going star formation activity in L1630N. The star formation efficiency
(SFE), star formation rate (SFR) and density of star formation of L1630N are
within the ranges estimated for galactic star forming regions by the Spitzer
"core to disk" and "Gould's Belt" surveys. However, the SFE and SFR are lower
than the average value measured in the Orion A cloud and, in particular, lower
than that in the southern regions of L1630. This might suggest different star
formation mechanisms within the L1630 cloud complex.Comment: 22 pages, 9 figure
The Chamaeleon II low-mass star-forming region: radial velocities, elemental abundances, and accretion properties
Radial velocities, elemental abundances, and accretion properties of members
of star-forming regions (SFRs) are important for understanding star and planet
formation. While infrared observations reveal the evolutionary status of the
disk, optical spectroscopy is fundamental to acquire information on the
properties of the central star and on the accretion characteristics. 2MASS
archive data and the Spitzer c2d survey of the Chamaeleon II dark cloud have
provided disk properties of a large number of young stars. We complement these
data with spectroscopy with the aim of providing physical stellar parameters
and accretion properties. We use FLAMES/UVES+GIRAFFE observations of 40 members
of Cha II to measure radial velocities through cross-correlation technique, Li
abundances by means of curves of growth, and for a suitable star elemental
abundances of Fe, Al, Si, Ca, Ti, and Ni using the code MOOG. From the
equivalent widths of the Halpha, Hbeta, and the HeI-5876, 6678, 7065 Angstrom
emission lines, we estimate the mass accretion rates, dMacc/dt, for all the
objects. We derive a radial velocity distribution for the Cha II stars
(=11.4+-2.0 km/s). We find dMacc/dt prop. to Mstar^1.3 and to Age^(-0.82)
in the 0.1-1.0 Msun mass regime, and a mean dMacc/dt for Cha II of ~7*10^(-10)
Msun/yr. We also establish a relationship between the HeI-7065 Angstrom line
emission and the accretion luminosity. The radial velocity distributions of
stars and gas in Cha II are consistent. The spread in dMacc/dt at a given
stellar mass is about one order of magnitude and can not be ascribed entirely
to short timescale variability. Analyzing the relation between dMacc/dt and the
colors in Spitzer and 2MASS bands, we find indications that the inner disk
changes from optically thick to optically thin at dMacc/dt~10^(-10) Msun/yr.
Finally, the disk fraction is consistent with the age of Cha II.Comment: 21 Pages, 15 Figures, 7 Tables. Accepted for publication in Astronomy
and Astrophysics. Abstract shortene
On the accretion properties of young stellar objects in the L1615/L1616 cometary cloud
We present the results of FLAMES/UVES and FLAMES/GIRAFFE spectroscopic
observations of 23 low-mass stars in the L1615/L1616 cometary cloud,
complemented with FORS2 and VIMOS spectroscopy of 31 additional stars in the
same cloud. L1615/L1616 is a cometary cloud where the star formation was
triggered by the impact of the massive stars in the Orion OB association. From
the measurements of the lithium abundance and radial velocity, we confirm the
membership of our sample to the cloud. We use the equivalent widths of the
H, H, and the HeI 5876, 6678, 7065
\AAemission lines to calculate the accretion luminosities, ,
and the mass accretion rates, . We find in L1615/L1616 a
fraction of accreting objects (), which is consistent with the
typical fraction of accretors in T associations of similar age ( Myr).
The mass accretion rate for these stars shows a trend with the mass of the
central object similar to that found for other star-forming regions, with a
spread at a given mass which depends on the evolutionary model used to derive
the stellar mass. Moreover, the behavior of the colors with indicates that strong accretors with dex show large excesses in the bands, as in previous
studies. We also conclude that the accretion properties of the L1615/L1616
members are similar to those of young stellar objects in T associations, like
Lupus.Comment: Accepted by Astronomy and Astrophysics. 17 pages, 11 figures, 6
table
Detection of delta Scuti-like pulsation in H254, a pre-main sequence F-type star in IC 348
We present time series observations of intermediate mass PMS stars belonging
to the young star cluster IC 348. The new data reveal that a young member of
the cluster, H254, undergoes periodic light variations with delta Scuti-like
characteristics. This occurrence provides an unambiguous evidence confirming
the prediction that intermediate-mass pre-main sequence (PMS) stars should
experience this transient instability during their approach to the
main-sequence. On the basis of the measured frequency f=7.406 c/d, we are able
to constrain the intrinsic stellar parameters of H254 by means of linear, non
adiabatic, radial pulsation models. The range of the resulting luminosity and
effective temperature permitted by the models is narrower than the
observational values. In particular, the pulsation analysis allows to derive an
independent estimate of the distance to IC 348 of about 320 pc. Further
observations could either confirm the monoperiodic nature of H254 or reveal the
presence of other frequencies.Comment: 7 pages, including 7 postscript figures, accepted for publication on
A&
X-Shooter spectroscopy of young stellar objects: V - Slow winds in T Tauri stars
Disks around T Tauri stars are known to lose mass, as best shown by the
profiles of forbidden emission lines of low ionization species. At least two
separate kinematic components have been identified, one characterised by
velocity shifts of tens to hundreds km/s (HVC) and one with much lower velocity
of few km/s (LVC). The HVC are convincingly associated to the emission of jets,
but the origin of the LVC is still unknown. In this paper we analyze the
forbidden line spectrum of a sample of 44 mostly low mass young stars in Lupus
and -Ori observed with the X-Shooter ESO spectrometer. We detect
forbidden line emission of [OI], [OII], [SII], [NI], and [NII], and
characterize the line profiles as LVC, blue-shifted HVC and red-shifted HVC. We
focus our study on the LVC. We show that there is a good correlation between
line luminosity and both L and the accretion luminosity (or the
mass-accretion rate) over a large interval of values (L L; L L;
M/yr). The lines show the presence of a slow
wind ( cm), warm (T K), mostly neutral. We estimate the mass of the emitting gas and
provide a value for the maximum volume it occupies. Both quantities increase
steeply with the stellar mass, from M and
AU for M M, to
M and AU for M M, respectively.
These results provide quite stringent constraints to wind models in low mass
young stars, that need to be explored further
X-Shooter spectroscopy of young stellar objects - VI - HI line decrements
Hydrogen recombination emission lines commonly observed in accreting young
stellar objects represent a powerful tracer for the gas conditions in the
circumstellar structures. Here we perform a study of the HI decrements and line
profiles, from the Balmer and Paschen lines detected in the X-Shooter spectra
of a homogeneous sample of 36 T Tauri stars in Lupus, the accretion and stellar
properties of which were already derived in a previous work. We aim to obtain
information on the gas physical conditions to derive a consistent picture of
the HI emission mechanisms in pre-main sequence low-mass stars. We have
empirically classified the sources based on their HI line profiles and
decrements. We identified four Balmer decrement types (classified as 1, 2, 3,
and 4) and three Paschen decrement types (A, B, and C), characterised by
different shapes. We first discussed the connection between the decrement types
and the source properties and then compared the observed decrements with
predictions from recently published local line excitation models. One third of
the objects show lines with narrow symmetric profiles, and present similar
Balmer and Paschen decrements (straight decrements, types 2 and A). Lines in
these sources are consistent with optically thin emission from gas with
hydrogen densities of order 10^9 cm^-3 and 5000<T<15000 K. These objects are
associated with low mass accretion rates. Type 4 (L-shaped) Balmer and type B
Paschen decrements are found in conjunction with very wide line profiles and
are characteristic of strong accretors, with optically thick emission from
high-density gas (log n_H > 11 cm^-3). Type 1 (curved) Balmer decrements are
observed only in three sub-luminous sources viewed edge-on, so we speculate
that these are actually reddened type 2 decrements. About 20% of the objects
present type 3 Balmer decrements (bumpy), which cannot be reproduced with
current models.Comment: 29 pages, accepted by A&
- âŠ