18 research outputs found
Photometric monitoring of the young star Par 1724 in Orion
We report new photometric observations of the 200000 year old naked weak-line
run-away T Tauri star Par 1724, located north of the Trapezium cluster in
Orion. We observed in the broad band filters B, V, R, and I using the 90cm
Dutch telescope on La Silla, the 80cm Wendelstein telescope, and a 25cm
telescope of the University Observatory Jena in Grossschwabhausen near Jena.
The photometric data in V and R are consistent with a 5.7 day rotation period
due to spots, as observed before between 1960ies and 2000. Also, for the first
time, we present evidence for a long-term 9 or 17.5 year cycle in photometric
data (V band) of such a young star, a cycle similar to that to of the Sun and
other active stars.Comment: AN in press (eps or ps files on
http://www.astro.uni-jena.de/Observations/gsh/gsh_papers.htm
Exploring Halo Substructure with Giant Stars. VI. Extended Distributions of Giant Stars Around the Carina Dwarf Spheroidal Galaxy -- How Reliable Are They?
The question of the existence of active tidal disruption around various dSph
galaxies remains controversial. That debate often centers on the nature (bound
vs. unbound) of extended populations of stars. However, the more fundamental
issue of the very existence of the extended populations is still contentious.
We present an evaluation of the debate centering on one particular dSph,
Carina, for which claims both for and against the existence of stars beyond the
King radius have been made. Our review includes an examination of all previous
studies bearing on the Carina radial profile and shows that the survey method
which achieves the highest detected dSph signal-to-background in the outer
parts of the galaxy is the Washington M, T2 + DDO51 (MTD) filter approach from
Paper II in this series. We then address statistical methods used to evaluate
the reliability of MTD surveys in the presence of photometric errors and for
which a new, a posteriori statistical analysis methodology is provided.
Finally, these statistical methods are tested by new spectroscopy of stars in
the MTD-selected Carina candidate sample. Of 74 candidate giants with follow-up
spectroscopy, the MTD technique identified 61 new Carina members, including 8
stars outside the King radius. From a sample of 29 stars not initially
identified as candidate Carina giants but that lie just outside of our
selection criteria, 12 have radial velocities consistent with membership,
including 5 extratidal stars. Carina is shown to have an extended population of
giant stars extending to a major axis radius of 40' (1.44x the nominal King
radius).Comment: 56 pages, 10 figures. Submitted to the Astronomical Journal, 2004 Sep
2
Membership Determination of Open Cluster M48 Based on BATC Thirteen-Band Photometry
Beijing-Arizona-Taiwan-Connecticut (BATC) multi-band photometric data in the
field of open cluster M48 are used to determine its membership. By comparing
observed spectral energy distributions (SEDs) of stars with theoretical ones,
membership probabilities of 750 stars with limiting magnitude of 15.0 in BATC
band ( \AA) are determined. 323 stars with membership
probabilities higher than 30% are considered as candidate members of M48.
Comparing membership probabilities of 229 common stars obtained by the present
method and the proper-motion based methods, a 80% agreement among these methods
is obtained.Comment: 27 pages,7figures, accepted for publication in PAS
The Absolute Proper Motion of Palomar 12: A Case for Tidal Capture from the Sagittarius Dwarf Spheroidal Galaxy
We have measured the absolute proper motion of the young globular cluster Pal
12 with respect to background galaxies, using plate material spanning a 40-year
time baseline, and measuring stars down to a magnitude V~22. The measured
absolute proper motion has an uncertainty of 0.3 mas/yr in each coordinate.
Pal 12's young age for a globular cluster led to the hypothesis that the
cluster originated in the Large Magellanic Cloud (LMC) and was later captured
by the Milky Way (Lin and Richer 1992). Here we investigate this hypothesis
using the complete kinematical data. We present the orbital characteristics of
Pal 12 and compare them with those of the LMC and Sagittarius dwarf galaxy
(Sgr). The present kinematical data suggest that, from the two parent
candidates for Pal 12, Sgr presents a more plausible case for the host galaxy
than the LMC.
We explore this scenario in the context of the uncertainties in the orbits
and using two different analyses: the direct comparison of the orbits of Pal 12
and Sgr as a function of time, and the analytical model of Sgr's tidal
disruption developed by Johnson (1998). We find that, within the present
uncertainties of the observables, this scenario is viable in both methods.
Moreover, both methods place this event at the same point in time. Our best
estimate of the time of Pal 12's tidal capture from Sgr is ~ 1.7 Gyr ago.Comment: 37 pages, 5 tables, 5 figures, accepted for publication in AJ, Oct.
200
A maximum likelihood method for fitting colour-magnitude diagrams
We present a maximum likelihood method for fitting two-dimensional model
distributions to stellar data in colour-magnitude space. This allows one to
include (for example) binary stars in an isochronal population. The method also
allows one to derive formal uncertainties for fitted parameters, and assess the
likelihood that a good fit has been found. We use the method to derive an age
of 38.5 +3.5/-6.5 Myrs and a true distance modulus of 7.79 +0.11/-0.05 mags
from the V vs V-I diagram of NGC2547 (the uncertainties are 67 percent
confidence limits, and the parameters are insensitive to the assumed binary
fraction). These values are consistent with those previously determined from
low-mass isochronal fitting, and are the first measurements to have
statistically meaningful uncertainties. The age is also consistent with the
lithium depletion age of NGC2547, and the HIPPARCOS distance to the cluster is
consistent with our value.
The method appears to be quite general and could be applied to any
N-dimensional dataset, with uncertainties in each dimension. However, it is
particularly useful when the data are sparse, in the sense that both the
typical uncertainties for a datapoint and the size of structure in the function
being fitted are small compared with the typical distance between datapoints.
In this case binning the data will lose resolution, whilst the method presented
here preserves it.
Software implementing the methods described in this paper is available from
http://www.astro.ex.ac.uk/people/timn/tau-squared/.Comment: 15 pages, 17 figure
Wide and deep near-UV (360nm) galaxy counts and the extragalactic background light with the Large Binocular Camera
Deep multicolour surveys are the main tool to explore the formation and
evolution of the faint galaxies which are beyond the spectroscopic limit with
the present technology. The photometric properties of these faint galaxies are
usually compared with current renditions of semianalytical models to provide
constraints on the fundamental physical processes involved in galaxy formation
and evolution, namely the mass assembly and the star formation. Galaxy counts
over large sky areas in the near-UV band are important because they are
difficult to obtain given the low efficiency of near-UV instrumentation, even
at 8m class telescopes. A large instrumental field of view helps in minimizing
the biases due to the cosmic variance. We have obtained deep images in the
360nm U band provided by the blue channel of the Large Binocular Camera at the
prime focus of the Large Binocular Telescope. We have derived over an area of
~0.4 sq. deg. the galaxy number counts down to U=27 in the Vega system
(corresponding to U=27.86 in the AB system) at a completeness level of 30%
reaching the faintest current limit for this wavelength and sky area. The shape
of the galaxy counts in the U band can be described by a double power-law, the
bright side being consistent with the shape of shallower surveys of comparable
or greater areas. The slope bends over significantly at U>23.5 ensuring the
convergence of the contribution by star forming galaxies to the EBL in the
near-UV band to a value which is more than 70% of the most recent upper limits
derived for this band. We have jointly compared our near-UV and K band counts
collected from the literature with few selected hierarchical CDM models
emphasizing critical issues in the physical description of the galaxy formation
and evolution.Comment: Accepted for publication in A&A. Uses aa.cls, 9 pages, 4 figures.
Citations update
The CARMENES search for exoplanets around M dwarfs High-resolution optical and near-infrared spectroscopy of 324 survey stars
The CARMENES radial velocity (RV) survey is observing 324 M dwarfs to search for any orbiting planets. In this paper, we present the survey sample by publishing one CARMENES spectrum for each M dwarf. These spectra cover the wavelength range 520â1710 nm at a resolution of at least R >80 000, and we measure its RV, Hα emission, and projected rotation velocity. We present an atlas of high-resolution M-dwarf spectra and compare the spectra to atmospheric models. To quantify the RV precision that can be achieved in low-mass stars over the CARMENES wavelength range, we analyze our empirical information on the RV precision from more than 6500 observations. We compare our high-resolution M-dwarf spectra to atmospheric models where we determine the spectroscopic RV information content, Q, and signal-to-noise ratio. We find that for all M-type dwarfs, the highest RV precision can be reached in the wavelength range 700â900 nm. Observations at longer wavelengths are equally precise only at the very latest spectral types (M8 and M9). We demonstrate that in this spectroscopic range, the large amount of absorption features compensates for the intrinsic faintness of an M7 star. To reach an RV precision of 1 m sâ1 in very low mass M dwarfs at longer wavelengths likely requires the use of a 10 m class telescope. For spectral types M6 and earlier, the combination of a red visual and a near-infrared spectrograph is ideal to search for low-mass planets and to distinguish between planets and stellar variability. At a 4 m class telescope, an instrument like CARMENES has the potential to push the RV precision well below the typical jitter level of 3â4 m sâ1
A giant exoplanet orbiting a very-low-mass star challenges planet formation models
Surveys have shown that super-Earth and Neptune-mass exoplanets are more frequent than gas giants around low-mass stars, as predicted by the core accretion theory of planet formation. We report the discovery of a giant planet around the very-low-mass star GJ 3512, as determined by optical and near-infrared radial-velocity observations. The planet has a minimum mass of 0.46 Jupiter masses, very high for such a small host star, and an eccentric 204-day orbit. Dynamical models show that the high eccentricity is most likely due to planet-planet interactions. We use simulations to demonstrate that the GJ 3512 planetary system challenges generally accepted formation theories, and that it puts constraints on the planet accretion and migration rates. Disk instabilities may be more efficient in forming planets than previously thought