167 research outputs found
A probable pre-main sequence chemically peculiar star in the open cluster Stock 16
We used the Ultraviolet and Visual Echelle Spectrograph of the ESO-Very Large
Telescope to obtain a high resolution and high signal-to-noise ratio spectrum
of Stock 16-12, an early-type star which previous Delta-a photometric
observations suggest being a chemically peculiar (CP) star. We used spectral
synthesis to perform a detailed abundance analysis obtaining an effective
temperature of 8400 +/- 400 K, a surface gravity of 4.1 +/- 0.4, a
microturbulence velocity of 3.4 +0.7/-0.3 km/s, and a projected rotational
velocity of 68 +/- 4 km/s. We provide photometric and spectroscopic evidence
showing the star is most likely a member of the young Stock 16 open cluster
(age 3-8 Myr). The probable cluster membership, the star's position in the
Hertzsprung-Russell diagram, and the found infrared excess strongly suggest the
star is still in the pre-main-sequence (PMS) phase. We used PMS evolutionary
tracks to determine the stellar mass, which ranges between 1.95 and 2.3 Msun,
depending upon the adopted spectroscopic or photometric data results.
Similarly, we obtained a stellar age ranging between 4 and 6 Myr, in agreement
with that of the cluster. Because the star's chemical abundance pattern
resembles well that known of main sequence CP metallic line (Am) stars, the
object sets important constraints to the diffusion theory. Additional
spectroscopic and spectropolarimetric data allowed us to conclude that the
object is probably a single non-magnetic star.Comment: Accepted for publication in MNRAS; 8 pages, 5 figures, 1 tabl
gamma Doradus pulsation in two pre-main sequence stars discovered by CoRoT
Pulsations in pre-main sequence stars have been discovered several times
within the last years. But nearly all of these pulsators are of delta
Scuti-type. gamma Doradus-type pulsation in young stars has been predicted by
theory, but lack observational evidence. We present the investigation of
variability caused by rotation and (gammaDoradus-type) pulsation in two
pre-main sequence members of the young open cluster NGC2264 using
high-precision time series photometry from the CoRoT satellite and dedicated
high-resolution spectroscopy. Time series photometry of NGC2264VAS20 and NGC
2264VAS87 was obtained by the CoRoT satellite during the dedicated short run
SRa01 in March 2008. NGC2264VAS87 was re-observed by CoRoT during the short run
SRa05 in December 2011 and January 2012. Frequency analysis was conducted using
Period04 and SigSpec. The spectral analysis was performed using equivalent
widths and spectral synthesis. The frequency analysis yielded 10 and 14
intrinsic frequencies for NGC2264VAS20 and NGC2264VAS 87, respectively, in the
range from 0 to 1.5c/d which are attributed to be caused by a combination of
rotation and pulsation. The effective temperatures were derived to be
6380150K for NGC2264VAS20 and 6220150K for NGC2264VAS87. Membership
of the two stars to the cluster is confirmed independently using X-ray fluxes,
radial velocity measurements and proper motions available in the literature.
The derived Li abundances of log n(Li)=3.34 and 3.54 for NGC2264VAS20 and
NGC2264VAS87, respectively, are in agreement with the Li abundance for other
stars in NGC2264 of similar Teff reported in the literature. We conclude that
the two objects are members of NGC2264 and therefore are in their pre-main
sequence evolutionary stage. Assuming that part of their variability is caused
by pulsation, these two stars might be the first pre-main sequence gamma
Doradus candidates.Comment: 11 pages, 10 figures, A&A accepte
Pre-main sequence variable stars in young open cluster NGC 1893
We present results of multi-epoch (fourteen nights during 2007-2010) -band
photometry of the cluster NGC 1893 region to identify photometric variable
stars in the cluster. The study identified a total of 53 stars showing
photometric variability. The members associated with the region are identified
on the basis of spectral energy distribution, two colour diagram and
colour-magnitude diagram. The ages and masses of the majority of
pre-main-sequence sources are found to be 5 Myr and in the range 0.5
4, respectively. These pre-main-sequence
sources hence could be T Tauri stars. We also determined the physical
parameters like disk mass and accretion rate from the spectral energy
distribution of these T Tauri stars. The periods of majority of the T Tauri
stars range from 0.1 to 20 day. The brightness of Classical T Tauri stars is
found to vary with larger amplitude in comparison to Weak line T Tauri stars.
It is found that the amplitude decreases with increase in mass, which could be
due to the dispersal of disks of massive stars.Comment: 15pages, 16 figures and 5 tables, Accepted for publication in MNRA
HD 41641: A classical Sct-type pulsator with chemical signatures of an Ap star
Among the known groups of pulsating stars, Sct stars are one of the
least understood. Theoretical models do not predict the oscillation frequencies
that observations reveal. Complete asteroseismic studies are necessary to
improve these models and better understand the internal structure of these
targets. We study the Sct star HD 41641 with the ultimate goal of
understanding its oscillation pattern. The target was simultaneously observed
by the CoRoT space telescope and the HARPS high-resolution spectrograph. The
photometric data set was analyzed with the software package PERIOD04, while
FAMIAS was used to analyze the line profile variations. The method of spectrum
synthesis was used for spectroscopically determining the fundamental
atmospheric parameters and individual chemical abundances. A total of 90
different frequencies was identified and analyzed. An unambiguous
identification of the azimuthal order of the surface geometry could only be
provided for the dominant p-mode, which was found to be a nonradial prograde
mode with m = +1. Using and , we estimated the mass,
radius, and evolutionary stage of HD 41641. We find HD 41641 to be a moderately
rotating, slightly evolved Sct star with subsolar overall atmospheric
metal content and unexpected chemical peculiarities. HD 41641 is a pure
Sct pulsator with p-mode frequencies in the range from 10 d to
20 d. This pulsating star presents chemical signatures of an Ap star and
rotational modulation due to surface inhomogeneities, which we consider
indirect evidence of the presence of a magnetic field.Comment: 11 pages, 11 figures, accepted for publication in A&
Investigating star formation in the young open cluster NGC 6383
By studying young open clusters, the mechanisms important for star formation
over several Myr can be examined. For example, accretion rate as a function of
rotational velocity can be investigated. Similarly, sequential star formation
triggered by massive stars with high mass-loss rates can be studied in detail.
We identified and characterized probable members of NGC 6383, as well as
determined cluster parameters. New Stromgren uvby CCD photometry, obtained by
us, is presented. This new data, together with Johnson UBV and 2MASS data in
the NIR, was used to investigate characteristics of pre- as well as zero age
main sequence cluster members. We present Stromgren uvby CCD photometry for 272
stars in the field of NGC 6383 and derive its reddening, E(b-y)=0.21(4)mag, as
well as distance, d=1.7(3)kpc from the Sun. Several stars with NIR excess and
objects in the domain of the classical Herbig Ae/Be and T Tauri stars were
detected. Two previously known variables were identified as rapidly-rotating
PMS stars. The field population is clearly separated from the probable members
in the color-magnitude diagram. NGC 6383 is a young open cluster, with an age
of less than 4 Myr, undergoing continuous star formation. True pre-main
sequence members might be found down to absolute magnitudes of +6mag, with a
variety of rotational velocities and stellar activities.Comment: 7 pages, 6 figures, accepted by A&
A search for transiting planets in the Pictoris system
The bright star Pictoris is a nearby young star with a
debris disk and gas giant exoplanet, Pictoris b, in a multi-decade
orbit around it. Both the planet's orbit and disk are almost edge-on to our
line of sight. We carry out a search for any transiting planets in the
Pictoris system with orbits of less than 30 days that are coplanar with the
planet Pictoris b. We search for a planetary transit using data from
the BRITE-Constellation nanosatellite BRITE-Heweliusz, analyzing the photometry
using the Box-Fitting Least Squares Algorithm (BLS). The sensitivity of the
method is verified by injection of artificial planetary transit signals using
the Bad-Ass Transit Model cAlculatioN (BATMAN) code. No planet was found in the
BRITE-Constellation data set. We rule out planets larger than 0.6
for periods of less than 5 days, larger than 0.75
for periods of less than 10 days, and larger than 1.05 for
periods of less than 20 days.Comment: 6 pages, 6 figures, 3 tables. Accepted for publication in A&
MOST photometry of the enigmatic PMS pulsator HD 142666
We present precise photometry of the pulsating Herbig Ae star HD 142666
obtained in two consecutive years with the MOST (Microvariability & Oscilations
of STars) satellite.
Previously, only a single pulsation period was known for HD 142666. The MOST
photometry reveals that HD 142666 is multi-periodic. However, the unique
identification of pulsation frequencies is complicated by the presence of
irregular variability caused by the star's circumstellar dust disk. The two
light curves obtained with MOST in 2006 and 2007 provided data of unprecedented
quality to study the pulsations in HD 142666 and also to monitor the
circumstellar variability.
We attribute 12 frequencies to pulsation. Model fits to the three frequencies
with the highest amplitudes lie well outside the uncertainty box for the star's
position in the HR diagram based on published values.
The models suggest that either (1) the published estimate of the luminosity
of HD 142666, based on a relation between circumstellar disk radius and stellar
luminosity, is too high and/or (2) additional physics such as mass accretion
may be needed in our models to accurately fit both the observed frequencies and
HD 142666's position in the HR diagram.Comment: 10 pages, 11 figures, accepted for publication by Astronomy and
Astrophysic
Monitoring young associations and open clusters with Kepler in two-wheel mode
We outline a proposal to use the Kepler spacecraft in two-wheel mode to
monitor a handful of young associations and open clusters, for a few weeks
each. Judging from the experience of similar projects using ground-based
telescopes and the CoRoT spacecraft, this program would transform our
understanding of early stellar evolution through the study of pulsations,
rotation, activity, the detection and characterisation of eclipsing binaries,
and the possible detection of transiting exoplanets. Importantly, Kepler's wide
field-of-view would enable key spatially extended, nearby regions to be
monitored in their entirety for the first time, and the proposed observations
would exploit unique synergies with the GAIA ESO spectroscopic survey and, in
the longer term, the GAIA mission itself. We also outline possible strategies
for optimising the photometric performance of Kepler in two-wheel mode by
modelling pixel sensitivity variations and other systematics.Comment: 10 pages, 6 figures, white paper submitted in response to NASA call
for community input for alternative science investigations for the Kepler
spacecraf
Searching for solar-like oscillations in pre-main sequence stars using APOLLO
In recent years, our understanding of solar-like oscillations from main
sequence to red giant stars has improved dramatically thanks to pristine data
collected from space telescopes. One of the remaining open questions focuses
around the observational identification of solar-like oscillations in pre-main
sequence stars.
We aim to develop an improved method to search for solar-like oscillations in
pre-main sequence stars and apply it to data collected by the Kepler K2
mission. Our software APOLLO includes a novel way to detect low signal-to-noise
ratio solar like oscillations in the presence of a high background level.
By calibrating our method using known solar-like oscillators from the main
Kepler mission, we apply it to T Tauri stars observed by Kepler K2 and identify
several candidate pre-main sequence solar-like oscillators. We find that our
method is robust even when applied to time-series of observational lengths as
short as those obtained with the TESS satellite in one sector. We identify EPIC
205375290 as a possible candidate for solar-like oscillations in a pre-main
sequence star with Hz. We also derive EPIC
205375290's fundamental parameters to be = 3670180 K, log
= 3.850.3, sin = 8 1 km s, and about solar
metallicity from a high-resolution spectrum obtained from the Keck archive.Comment: 14 pages, 13 figure
Search for pulsating PMS stars in NGC 6383
A search for pulsating pre-main sequence (PMS) stars was performed in the
young open cluster NGC 6383 using CCD time series photometry in Johnson B & V
filters. With an age of only ~1.7 million years all cluster members later than
spectral type A0 have not reached the ZAMS yet, hence being ideal candidates
for investigating PMS pulsation among A and F type stars. In total 286 stars
have been analyzed using classical Fourier techniques. From about a dozen of
stars within the boundaries of the classical instability strip, two stars were
found to pulsate: NGC 6383 #170, with five frequencies simultaneously, and NGC
6383 #198, with a single frequency. In addition, NGC 6383 #152 is a suspected
PMS variable star, but our data remain inconclusive. Linear, non-adiabatic
models assuming PMS evolutionary phase and purely radial pulsation were
calculated for the two new PMS pulsators. NGC 6383 #170 appears to pulsate
radially in third and fifth overtones, while the other three frequencies seem
to be of non-radial nature. NGC 6383 #198 pulsates monoperiodically, most
probably in the third radial overtone. Magnitudes and B-V colours were
available in the literature for only one third of all stars and we used them
for calibrating the remaining.Comment: 12 pages, 11 figures, accepted by MNRA
- …