401 research outputs found
A very massive runaway star from Cygnus OB2
Aims: We analyze the available information on the star BD+43 3654 to
investigate the possibility that it may have had its origin in the massive OB
association Cygnus OB2.
Methods: We present new spectroscopic observations allowing a reliable
spectral classification of the star, and discuss existing MSX observations of
its associated bow shock and astrometric information not previously studied.
Results: Our observations reveal that BD+43 3654 is a very early and luminous
star of spectral type O4If, with an estimated mass of (70 +/- 15) solar masses
and an age of about 1.6 Myr. The high spatial resolution of the MSX
observations allows us to determine its direction of motion in the plane of the
sky by means of the symmetry axis of the well-defined bow shock, which matches
well the orientation expected from the proper motion. Tracing back its path
across the sky we find that BD+43 3654 was located near the central, densest
region of Cygnus OB2 at a time in the past similar to its estimated age.
Conclusions: BD+43 3654 turns out to be one of the three most massive runaway
stars known, and it most likely formed in the central region of Cygnus OB2. A
runaway formation mechanism by means of dynamical ejection is consistent with
our results.Comment: Accepted by Astronomy and Astrophysics (letters); 5 pages, 3 figure
Oxygen and silicon abundances in Cygnus OB2: Chemical homogeneity in a sample of OB slow rotators
Cygnus OB2 is a rich OB association in the Galaxy which has experienced
intense star formation in the last 20-25 Myr. Its stellar population shows a
correlation between age and Galactic longitude. Exploring the chemical
composition of its stellar content we will be able to check the degree of
homogeneity of the natal molecular cloud and possible effects of
self-enrichment processes. Our aim is to determine silicon and oxygen
abundances for a sample of eight early-type slow rotators in Cygnus OB2 in
order to check possible inhomogeneities across the whole association and
whether there exists a correlation of chemical composition with Galactic
longitude. We have performed a spectroscopic analysis of a sample of late O and
early B stars with low rotational velocity, which have been chosen so as to
cover the whole association area. We have carried out an analysis based on
equivalent widths of metal lines, the wings of the H Balmer lines and FASTWIND
stellar atmosphere models to determine their stellar fundamental parameters as
well as the silicon and oxygen surface abundances. We derive a rather
homogeneous distribution of silicon and oxygen abundances across the region,
with average values of 12+log(Si/H)=7.530.08 dex and
12+log(O/H)=8.650.12 dex. We find a homogeneous chemical composition in
Cygnus OB2 with no clear evidence for significant chemical self-enrichment,
despite indications of strong stellar winds and possible supernovae during the
history of the region. Comparison with different scenarios of chemical
enrichment by stellar winds and supernovae point to star forming efficiencies
not significantly above 10%. The degree of homogeneity that we find is
consistent with the observed Milky Way oxygen gradient based on HII regions. We
also find that the oxygen scatter within Cygnus OB2 is at least of the same
order than among HII regions at similar Galactocentric distance.Comment: 15 pages, 7 figures, accepted for publication in Astronomy &
Astrophysic
The Formation and Early Evolution of Low-mass Stars and Brown Dwarfs
The discovery of large numbers of young low-mass stars and brown dwarfs over
the last decade has made it possible to investigate star formation and early
evolution in a previously unexplored mass regime. In this review, we begin by
describing surveys for low-mass members of nearby associations, open clusters,
star-forming regions and the methods used to characterize their stellar
properties. We then use observations of these populations to test theories of
star formation and evolution at low masses. For comparison to the formation
models, we consider the initial mass function, stellar multiplicity,
circumstellar disks, protostellar characteristics, and kinematic and spatial
distributions at birth for low-mass stars and brown dwarfs. To test the
evolutionary models, we focus on measurements of dynamical masses and empirical
Hertzsprung-Russell diagrams for young brown dwarfs and planetary companions.Comment: Final published version at http://www.annualreviews.org/journal/astr
Exploring Brown Dwarf Disks
We discuss the spectral energy distribution of three very low mass objects in
Chamaeleon I for which ground-based spectroscopy and photometry as well as ISO
measurements in the mid-infrared are available (Comeron et al. 2000; Persi et
al. 2000). One of these stars (Cha Halpha1) is a bona-fide brown dwarf, with
mass 0.04-0.05 Msun. We show that the observed emission is very well described
by models of circumstellar disks identical to those associated to T Tauri
stars, scaled down to keep the ratio of the disk-to-star mass constant and to
the appropriate stellar parameters. This result provides a first indication
that the formation mechanism of T Tauri stars (via core contraction and
formation of an accretion disk) extends to objects in the brown dwarf mass
range.Comment: 4 pages, 4 figures, A&A Letters in pres
The extended population associated with W40
W40 is a heavily obscured bipolar HII region projected in the direction of
the Aquila Rift and ionized by hot stars in a central, partly embedded cluster.
The study of the cluster and its surroundings has been greatly hampered thus
far by the strong extinction in the region. We use the Gaia eDR3 catalog to
establish astrometric membership criteria based on the population of the W40
central cluster, reassess the distance of the region, and identify in this way
new members, both inside and outside the cluster. We obtain visible
spectroscopy in the red spectral region to classify both known and new members,
complemented with Gaia and Spitzer photometry to assess the evolutionary status
of the stellar population. We derive a high-confidence geometric distance to
the W40 region of 502 pc 4 pc and confirm the presence of a comoving
extended population of stars at the same distance, spreading over the whole
projected area of the HII region and beyond. Spectral classifications are
presented for 21 members of the W40 region, 10 of them belonging to the central
cluster. One of the newly identified B stars in the extended population is
clearly interacting with the shell surrounding the HII region, giving rise to a
small arc-shaped nebula that traces a bow shock. The infrared excess properties
suggest that the extended population is significantly older ( Myr) than
the W40 central cluster ( Myr). The area currently occupied by the W40 HII
region and its surroundings has a history of star formation extending at least
several million years in the past, of which the formation of the W40 central
cluster and the subsequent HII region is one of the latest episodes. The newly
determined distance suggests that W40 is behind, and physically detached from,
a pervasive large dust layer which is some 60 pc foreground to it as determined
by previous studies.Comment: Submitted to Astronomy and Astrophysic
Accretion in Brown Dwarfs: an Infrared View
This paper presents a study of the accretion properties of 19 very low mass
objects (Mstar .01-0.1 Msun) in the regions Chamaeleon I and rho-Oph For 8
objects we obtained high resolution Halpha profiles and determined mass
accretion rate Macc and accretion luminosity Lacc. Pabeta is detected in
emission in 7 of the 10 rho-Oph objects, but only in one in Cha I. Using
objects for which we have both a determination of Lacc from Halpha and a Pabeta
detection, we show that the correlation between the Pabeta luminosity and
luminosity Lacc, found by Muzerolle et al. (1998) for T Tauri stars in Taurus,
extends to objects with mass approx 0.03 Msun; L(Pab) can be used to measure
Lacc also in the substellar regime. The results were less conclusive for
Brgamma, which was detected only in 2 objects, neither of which had an Halpha
estimate of Macc. Using the relation between L(Pab) and Lacc we determined the
accretion rate for all the objects in our sample (including those with no
Halpha spectrum), more than doubling the number of substellar objects with
known Macc. When plotted as a function of the mass of the central object
together with data from the literature, our results confirm the trend of lower
Macc for lower Mstar, although with a large spread. Some of the spread is
probably due to an age effect; our very young objects in rho-Oph have on
average an accretion rate at least one order of magnitude higher than objects
of similar mass in older regions. As a side product, we found that the width of
Halpha measured at 10% peak intensity is not only a qualitative indicator of
accretion, but can be used to obtain a quantitative estimate of Macc over a
large mass range, from T Tauri stars to brown dwarfs. Finally, we found that
some of our objects show evidence of mass-loss.Comment: 11 pages, 8 figures, A&A in pres
The historical record of massive star formation in Cygnus
The Cygnus region, which dominates the local spiral arm of the Galaxy, is one
of the nearest complexes of massive star formation. Its massive stellar
content, regions of ongoing star formation, and molecular gas have been studied
in detail. However, little is known of the history of the region beyond the
past 10 Myr. The brightness and spectroscopic characteristics of red
supergiants make it easy to identify them and build up a virtually complete
sample of such stars at the distance of the Cygnus region, thus providing a
record of massive star formation extending several tens of Myr into the past, a
period inaccessible through the O and early B stars observable at present. We
have made a selection of a sample of bright, red stars in an area of 84 square
degrees covering the whole present extension of the Cygnus region. We have
obtained spectroscopy in the red visible range allowing an accurate,
homogeneous spectral classification as well as a reliable separation between
supergiants and other cool stars. Our data are complemented with Gaia Data
Release 2 astrometric data. We have identified 29 red supergiants in the area,
17 of which had not been previously classified as supergiants. Twenty-four of
the 29 most likely belong to the Cygnus region and four of the remaining to the
Perseus arm. We have used their derived luminosities and masses to infer the
star formation history of the region. Intense massive star formation activity
is found to have started approximately 15 Myr ago, and we find evidence for two
other episodes, one taking place between 20 and 30 Myr ago and another one
having ended approximately 40 Myr ago. There are small but significant
differences between the kinematic properties of red supergiants younger or
older then 20 Myr, hinting that stars of the older group were formed outside
the precursor of the present Cygnus complex, possibly in the Sagittarius-Carina
arm.Comment: Accepted for publication in Astronomy and Astrophysic
X-Shooter spectroscopy of young stellar objects: II. Impact of chromospheric emission on accretion rate estimates
Context. The lack of knowledge of photospheric parameters and the level of
chromospheric activity in young low-mass pre-main sequence stars introduces
uncertainties when measuring mass accretion rates in accreting (Class II) Young
Stellar Objects. A detailed investigation of the effect of chromospheric
emission on the estimates of mass accretion rate in young low-mass stars is
still missing. This can be undertaken using samples of young diskless (Class
III) K and M-type stars. Aims. Our goal is to measure the chromospheric
activity of Class III pre main sequence stars to determine its effect on the
estimates of accretion luminosity (Lacc) and mass accretion rate (Macc) in
young stellar objects with disks. Methods. Using VLT/X-Shooter spectra we have
analyzed a sample of 24 non-accreting young stellar objects of spectral type
between K5 and M9.5. We identify the main emission lines normally used as
tracers of accretion in Class II objects, and we determine their fluxes in
order to estimate the contribution of the chromospheric activity to the line
luminosity. Results. We have used the relationships between line luminosity and
accretion luminosity derived in the literature for Class II objects to evaluate
the impact of chromospheric activity on the accretion rate measurements. We
find that the typical chromospheric activity would bias the derived accretion
luminosity by Lacc,noise< 10-3Lsun, with a strong dependence with the Teff of
the objects. The noise on Macc depends on stellar mass and age, and the typical
values of log(Macc,noise) range between -9.2 to -11.6Msun/yr. Conclusions.
Values of Lacc< 10-3Lsun obtained in accreting low-mass pre main sequence stars
through line luminosity should be treated with caution as the line emission may
be dominated by the contribution of chromospheric activity.Comment: accepted for publication in Astronomy & Astrophysic
High Moisture Sorghum Grain Silage: Effects of Tannin Content and Urea Treatment on the Performance of Dairy Cows
Grain sorghum silage tannin content effect was evaluated on milk production and chemical composition of Argentinean Holstein bred cows, and it are described dry matter (DM) and crude protein (CP) in situ digestion parameters and effective degradability. The base diet was constituted by alfalfa pasture, maize silage, and a protein – mineral supplement, differing in three treatments based on high moisture grain silage characteristic: LTS= Low tannin grain sorghum, HTS= High tannin grain sorghum, and HTSu= High tannin sorghum plus the addition of urea. Grain sorghum silage tannin content affects milk production, without significant alteration of chemical composition. Beside the effect of urea addition on grain sorghum tannin content, improvement in animal response was only moderate
X-Shooter spectroscopy of young stellar objects: IV -- Accretion in low-mass stars and sub-stellar objects in Lupus
We present X-Shooter/VLT observations of a sample of 36 accreting low-mass
stellar and sub-stellar objects (YSOs) in the Lupus star forming region,
spanning a range in mass from ~0.03 to ~1.2Msun, but mostly with 0.1Msun <
Mstar < 0.5Msun. Our aim is twofold: firstly, analyse the relationship between
excess-continuum and line emission accretion diagnostics, and, secondly, to
investigate the accretion properties in terms of the physical properties of the
central object. The accretion luminosity (Lacc), and from it the accretion rate
(Macc), is derived by modelling the excess emission, from the UV to the
near-IR, as the continuum emission of a slab of hydrogen. The flux and
luminosity (Ll) of a large number of emission lines of H, He, CaII, etc.,
observed simultaneously in the range from ~330nm to 2500nm, were computed. The
luminosity of all the lines is well correlated with Lacc. We provide empirical
relationships between Lacc and the luminosity of 39 emission lines, which have
a lower dispersion as compared to previous relationships in the literature. Our
measurements extend the Pab and Brg relationships to Lacc values about two
orders of magnitude lower than those reported in previous studies. We confirm
that different methodologies to measure Lacc and Macc yield significantly
different results: Ha line profile modelling may underestimate Macc by 0.6 to
0.8dex with respect to Macc derived from continuum-excess measures. Such
differences may explain the likely spurious bi-modal relationships between Macc
and other YSOs properties reported in the literature. We derive Macc in the
range 2e-12 -- 4e-8 Msun/yr and conclude that Macc is proportional to
Mstar^1.8(+/-0.2), with a dispersion lower by a factor of about 2 than in
previous studies. A number of properties indicate that the physical conditions
of the accreting gas are similar over more than 5 orders of magnitude in Macc
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