105 research outputs found
Transiting exoplanets from the CoRoT space mission IX. CoRoT-6b: a transiting `hot Jupiter' planet in an 8.9d orbit around a low-metallicity star
The CoRoT satellite exoplanetary team announces its sixth transiting planet
in this paper. We describe and discuss the satellite observations as well as
the complementary ground-based observations - photometric and spectroscopic -
carried out to assess the planetary nature of the object and determine its
specific physical parameters. The discovery reported here is a `hot Jupiter'
planet in an 8.9d orbit, 18 stellar radii, or 0.08 AU, away from its primary
star, which is a solar-type star (F9V) with an estimated age of 3.0 Gyr. The
planet mass is close to 3 times that of Jupiter. The star has a metallicity of
0.2 dex lower than the Sun, and a relatively high Li abundance. While
thelightcurveindicatesamuchhigherlevelof activity than, e.g., the Sun, there is
no sign of activity spectroscopically in e.g., the [Ca ] H&K lines
Search for associations containing young stars. III- Ages and Li abundances
Our study is a follow-up of the SACY project, an extended survey in the
Southern Hemisphere targeted to search for young nearby associations. Nine
associations have either been newly identified, or had their member list better
defined. These associations, with ages between about 6 Myr and 70 Myr, form an
excellent sample to study the Li depletion in the pre-main sequence (PMS)
evolution. We investigate the use of Li abundances as an independent clock to
constrain the PMS evolution. We have calculated the LTE Li abundances for 376
members of different young associations. In addition we considered the effects
of their projected stellar rotation. We present the Li depletion as function of
age in the first hundred million years for the first time for the most extended
sample of Li abundances in young stellar associations. A clear Li depletion can
be measured in the temperature range from 5000K to 3500K for the age span
covered by these nine associations. The age sequence based on the Li-clock
agrees well with the isochronal ages, Cha association being the only
possible exception. The lithium depletion patterns for those associations
resemble those of the young open clusters, strengthening the notion that the
members proposed for these loose young associations have indeed a common
physical origin. The observed scatter in the Li abundances hampers the use of
Li to determine reliable ages for individual stars. Rotation velocities above
20 km s seem to inhibit the Li depletion.Comment: 16 pgs., 7 figs To be published by Astronomy and Astrophysics
(accepted
Starspots on the fastest rotators in the Beta Pic moving group
Aims: We carried out high-resolution spectroscopy and BV(I)_C photometric
monitoring of the two fastest late-type rotators in the nearby Beta Pictoris
moving group, HD199143 (F7V) and CD-641208 (K7V). The motivation for this work
is to investigate the rotation periods and photospheric spot patterns of these
very young stars, with a longer term view to probing the evolution of rotation
and magnetic activity during the early phases of main-sequence evolution. We
also aim to derive information on key physical parameters, such as rotational
velocity and rotation period. Methods: We applied maximum entropy (ME) and
Tikhonov regularizing (TR) criteria to derive the surface spot map
distributions of the optical modulation observed in HD199143 (F7 V) and
CD-641208 (K7V). We also used cross-correlation techniques to determine stellar
parameters such as radial velocities and rotational velocities. Lomb-Scargle
periodograms were used to obtain the rotational periods from differential
magnitude time series. Results: We find periods and inclinations of 0.356 days
and 21.5deg for HD199143, and 0.355 days and 50.1deg for CD-641208. The spot
maps of HD199143 obtained from the ME and TR methods are very similar, although
the latter gives a smoother distribution of the filling factor. Maps obtained
at two different epochs three weeks apart show a remarkable increase in spot
coverage amounting to ~7% of the surface of the photosphere over a time period
of only ~20 days. The spot maps of CD-641208 from the two methods show good
longitudinal agreement, whereas the latitude range of the spots is extended to
cover the whole visible hemisphere in the TR map. The distributions obtained
from the first light curve of HD199143 show the presence of an extended and
asymmetric active longitude with the maximum filling factor at longitude
~325degree.Comment: Accepted by A&A. 13 pages, 13 figures (4 online included), 5 Table
On the age of the TW Hydrae Association and 2M1207334-393254
We have estimated the age of the young moving group TW Hydrae Association, a
cohort of a few dozen stars and brown dwarfs located near the Sun which share
the same kinematic properties and, presumably, the same origin and age. The
chronology has been determined by analyzing different properties (magnitudes,
colors, activity, lithium) of its members and comparing them with several
well-known star forming regions and open clusters, as well as theoretical
models. In addition, by using medium-resolution optical spectra of two M8
members of the association (2M1139 and 2M1207 -an accreting brown dwarf with a
planetary mass companion), we have derived spectral types and measured H(alpha)
and lithium equivalent widths. We have also estimated their effective
temperature and gravity, which were used to produce an independent age
estimation for these two brown dwarfs. We have also collected spectra of
2M1315, a candidate member with a L5 spectral type and measured its H(alpha)
equivalent width. Our age estimate for the association, 10 Myr (in the range
3-20 Myr), agrees with previous values cited in the literature. In the case of
the two brown dwarfs, we have derived an age of 15 Myr (in the range 5-30 Myr),
which also agree with our estimate for the whole group. We compared our results
with recent articles published on the same subject using other techniques, and
discuss the limits of the age-dating techniques.Comment: Astronomy & Astrophysics, accepte
Investigating potential planetary nebula/cluster pairs
Fundamental parameters characterizing the end-state of intermediate-mass
stars may be constrained by discovering planetary nebulae (PNe) in open
clusters (OCs). Cluster membership may be exploited to establish the distance,
luminosity, age, and physical size for PNe, and the intrinsic luminosity and
mass of its central star. Four potential PN-OC associations were investigated,
to assess the cluster membership for the PNe. Radial velocities were measured
from intermediate-resolution optical spectra, complemented with previous
estimates in the literature. When the radial velocity study supported the PN/OC
association, we analyzed if other parameters (e.g., age, distance, reddening,
central star brightness) were consistent with this conclusion. Our measurements
imply that the PNe VBe3 and HeFa1 are not members of the OCs NGC5999 and
NGC6067, respectively, and likely belong to the background bulge population.
Conversely, consistent radial velocities indicate that NGC2452/NGC2453 could be
associated, but our results are not conclusive and additional observations are
warranted. Finally, we demonstrate that all the available information point to
He2-86 being a young, highly internally obscured PN member of NGC4463. New
near-infrared photometry acquired via the Vista Variables in the Via Lactea ESO
public survey was used in tandem with existing UBV photometry to measure the
distance, reddening, and age of NGC4463, finding d=1.55+-0.10 kpc,
E(B-V)=0.41+-0.02, and tau=65+-10 Myr, respectively. The same values should be
adopted for the PN if the proposed cluster membership will be confirmed.Comment: Accepted for publication in A&
Identifying birth places of young isolated neutron stars
Young isolated radio-quiet neutron stars are still hot enough to be
detectable at X-ray and optical wavelengths due to their thermal emission and
can hence probe cooling curves. An identification of their birth sites can
constrain their age. For that reason we try to identify the parent associations
for four of the so-called Magnificent Seven neutron stars for which proper
motion and distance estimates are available. We are tracing back in time each
neutron star and possible birth association centre to find close encounters.
The associated time of the encounter expresses the kinematic age of the neutron
star which can be compared to its characteristic spin-down age. Owing to
observational uncertainties in the input data, we use Monte-Carlo simulations
and evaluate the outcome of our calculations statistically. RX J1856.5-3754
most probably originated from the Upper Scorpius association about 0.3 Myr ago.
RX 0720.4-3125 was either born in the young local association TWA about 0.4 Myr
ago or in Tr 10 0.5 Myr in the past. Also RX J1605.3+3249 and RBS 1223 seem to
come from a nearby young association such as the Sco-Cen complex or the
extended Corona-Australis association. For RBS 1223 also a birth in Sct OB2 is
possible. We also give constraints on the observables as well as on the radial
velocity of the neutron star. Given the birth association, its age and the
flight time of the neutron star, we estimate the mass of the progenitor star.
Some of the potential supernovae were located very nearby (<100pc) and thus
should have contributed to the 10Be and 60Fe material found in the Earth's
crust. In addition we reinvestigate the previously suggested neutron star/
runaway pair PSR B1929+10/ zeta Ophiuchi and conclude that it is very likely
that both objects were ejected during the same supernova event.Comment: 14 figures, 13 table
On the recent star formation history of the Milky Way disk
We have derived the star formation history of the Milky Way disk over the
last 2 Gyr from the age distribution diagram of a large sample of open clusters
comprising more than 580 objects. By interpreting the age distribution diagram
using numerical results from an extensive library of N-body calculations
carried out during the last ten years, we reconstruct the recent star formation
history of the Milky Way disk. Our analysis suggests that superimposed on a
relatively small level of constant star formation activity mainly in small-N
star clusters, the star formation rate has experienced at least 5 episodes of
enhanced star formation lasting about 0.2 Gyr with production of larger
clusters. This cyclic behavior seems to show a period of 0.4+/-0.1 Gyr.Comment: Abridged abstract. Accepted by New Astronomy. Major changes. A number
of figures have been added in order to improve the discussion on error
The Age-Activity-Rotation Relationship in Solar-Type Stars
Measured from high-resolution spectra, we present Ca II K line chromospheric
fluxes in 35 G dwarf stars of 5 open clusters to determine the
age-activity-rotation relationship from the young Hyades and Praesepe (0.6 Gyr)
to the old M 67 (~ 4.5 Gyr) through the two intermediate age clusters IC 4651
and NGC 3680 (~ 1.7 Gyr). Our data show that a dramatic decrease in
chromospheric activity takes place among solar stars between the Hyades and the
IC4651 age, in about 1 Gyr. Afterwards, activity remains virtually constant for
more than 3 Gyr. We briefly investigate the impact of this result on the age
determinations of field G dwarfs in the solar neighborhood; the two main
conclusions are that a consistent group of 'young' stars (i.e. as active as
Hyades stars) is present, and that it is virtually impossible to give accurate
chromospheric ages for stars older than ~ 2 Gyr. The observed abrupt decline in
activity explains very well the Vaughan-Preston gap.Comment: 17 pages, 10 figures. Accepted for publication in Astronomy &
Astrophysic
Titan's cold case files - Outstanding questions after Cassini-Huygens
Abstract The entry of the Cassini-Huygens spacecraft into orbit around Saturn in July 2004 marked the start of a golden era in the exploration of Titan, Saturn's giant moon. During the Prime Mission (2004â2008), ground-breaking discoveries were made by the Cassini orbiter including the equatorial dune fields (flyby T3, 2005), northern lakes and seas (T16, 2006), and the large positive and negative ions (T16 & T18, 2006), to name a few. In 2005 the Huygens probe descended through Titan's atmosphere, taking the first close-up pictures of the surface, including large networks of dendritic channels leading to a dried-up seabed, and also obtaining detailed profiles of temperature and gas composition during the atmospheric descent. The discoveries continued through the Equinox Mission (2008â2010) and Solstice Mission (2010â2017) totaling 127 targeted flybys of Titan in all. Now at the end of the mission, we are able to look back on the high-level scientific questions from the start of the mission, and assess the progress that has been made towards answering these. At the same time, new scientific questions regarding Titan have emerged from the discoveries that have been made. In this paper we review a cross-section of important scientific questions that remain partially or completely unanswered, ranging from Titan's deep interior to the exosphere. Our intention is to help formulate the science goals for the next generation of planetary missions to Titan, and to stimulate new experimental, observational and theoretical investigations in the interim
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