29 research outputs found
The radius and effective temperature of the binary Ap star beta CrB from CHARA/FLUOR and VLT/NACO observations
The prospects for using asteroseismology of rapidly oscillating Ap (roAp)
stars are hampered by the large uncertainty in fundamental stellar parameters.
Results in the literature for the effective temperature (Teff) often span a
range of 1000 K. Our goal is to reduce systematic errors and improve the Teff
calibration of Ap stars based on new interferometric measurements. We obtained
long-baseline interferometric observations of beta CrB using the CHARA/FLUOR
instrument. To disentangle the flux contributions of the two components of this
binary star, we obtained VLT/NACO adaptive optics images. We determined limb
darkened angular diameters of 0.699+-0.017 mas for beta CrB A (from
interferometry) and 0.415+-0.017 mas for beta CrB B (from surface brightness-
color relations), corresponding to radii of 2.63+-0.09 Rsun (3.4 percent
uncertainty) and 1.56+-0.07 Rsun (4.5 percent). The combined bolometric flux of
the A and B components was determined from satellite UV data, spectrophotometry
in the visible and broadband data in the infrared. The flux from the B
component constitutes 16+-4 percent of the total flux and was determined by
fitting an ATLAS9 model atmosphere to the broad-band NACO J and K magnitudes.
Combining the flux of the A component with its measured angular diameter, we
determine the effective temperature Teff(A) = 7980+-180 K (2.3 percent). Our
new interferometric and imaging data enable a nearly model-independent
determination of the effective temperature of beta CrB A. Including our recent
study of alpha Cir, we now have direct Teff measurements of two of the
brightest roAp stars, providing a strong benchmark for an improved calibration
of the Teff scale for Ap stars. This will support the use of potentially strong
constraints imposed by asteroseismic studies of roAp stars.Comment: 7 pages, accepted by A&
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&
Absolute dimensions of eclipsing binaries. XXIX. The Am-type systems SW Canis Majoris and HW Canis Majoris
CONTEXT: Accurate physical properties of eclipsing stars provide important
constraints on models of stellar structure and evolution, especially when
combined with spectroscopic information on their chemical composition.
Empirical calibrations of the data also lead to accurate mass and radius
estimates for exoplanet host stars. Finally, accurate data for unusual stellar
subtypes, such as Am stars, also help to unravel the cause(s) of their
peculiarities. AIMS: We aim to determine the masses, radii, effective
temperatures, detailed chemical composition and rotational speeds for the
Am-type eclipsing binaries SW CMa (A4-5m) and HW CMa (A6m) and compare them
with similar normal stars. METHODS: Accurate radial velocities from the Digital
Speedometers of the Harvard-Smithsonian Center for Astrophysics were combined
with previously published uvby photometry to determine precise physical
parameters for the four stars. A detailed abundance analysis was performed from
high-resolution spectra obtained with the Nordic Optical Telescope (La Palma).
RESULTS: We find the masses of the (relatively evolved) stars in SW CMa to be
2.10 and 2.24 solar masses, with radii of 2.50 and 3.01 solar radii, while the
(essentially zero-age) stars in HW CMa have masses of 1.72 and 1.78 solar
masses, radii of 1.64 and 1.66 solar radii -- all with errors well below 2%.
Detailed atmospheric abundances for one or both components were determined for
14 elements in SW CMa ([Fe/H] = +0.49/+0.61 dex) and 16 in HW CMa ([Fe/H] =
+0.33/+0.32 dex); both abundance patterns are characteristic of metallic-line
stars. Both systems are well fit by current stellar evolution models for
assumed bulk abundances of [Fe/H] = +0.05 and +0.23, respectively ([alpha/Fe] =
0.0), and ages of about 700 Myr and 160 Myr.Comment: 26 pages including figures, tables, and appendices. To appear in
Astronomy & Astrophysic
The Gaia-ESO Survey::the present-day radial metallicity distribution of the Galactic disc probed by pre-main-sequence clusters
Context. The radial metallicity distribution in the Galactic thin disc represents a crucial constraint for modelling disc formation and evolution. Open star clusters allow us to derive both the radial metallicity distribution and its evolution over time.
Aims. In this paper we perform the first investigation of the present-day radial metallicity distribution based on [Fe/H] determinations in late type members of pre-main-sequence clusters. Because of their youth, these clusters are therefore essential for tracing the current interstellar medium metallicity.
Methods. We used the products of the Gaia-ESO Survey analysis of 12 young regions (age < 100 Myr), covering Galactocentric distances from 6.67 to 8.70 kpc. For the first time, we derived the metal content of star forming regions farther than 500 pc from the Sun. Median metallicities were determined through samples of reliable cluster members. For ten clusters the membership analysis is discussed in the present paper, while for other two clusters (i.e. Chamaeleon I and Gamma Velorum) we adopted the members identified in our previous works.
Results. All the pre-main-sequence clusters considered in this paper have close-to-solar or slightly sub-solar metallicities. The radial metallicity distribution traced by these clusters is almost flat, with the innermost star forming regions having [Fe/H] values that are 0.10−0.15 dex lower than the majority of the older clusters located at similar Galactocentric radii.
Conclusions. This homogeneous study of the present-day radial metallicity distribution in the Galactic thin disc favours models that predict a flattening of the radial gradient over time. On the other hand, the decrease of the average [Fe/H] at young ages is not easily explained by the models. Our results reveal a complex interplay of several processes (e.g. star formation activity, initial mass function, supernova yields, gas flows) that controlled the recent evolution of the Milky Way
Gaia Data Release 1: Open cluster astrometry: performance, limitations, and future prospects
Context. The first Gaia Data Release contains the Tycho-Gaia Astrometric Solution (TGAS). This is a subset of about 2 million stars for which, besides the position and photometry, the proper motion and parallax are calculated using Hipparcos and Tycho-2 positions in 1991.25 as prior information.Aims. We investigate the scientific potential and limitations of the TGAS component by means of the astrometric data for open clusters.Methods. Mean cluster parallax and proper motion values are derived taking into account the error correlations within the astrometric solutions for individual stars, an estimate of the internal velocity dispersion in the cluster, and, where relevant, the effects of the depth of the cluster along the line of sight. Internal consistency of the TGAS data is assessed.Results. Values given for standard uncertainties are still inaccurate and may lead to unrealistic unit-weight standard deviations of least squares solutions for cluster parameters. Reconstructed mean cluster parallax and proper motion values are generally in very good agreement with earlier HIPPARCOS-based determination, although the Gaia mean parallax for the Pleiades is a significant exception. We have no current explanation for that discrepancy. Most clusters are observed to extend to nearly 15 pc from the cluster centre, and it will be up to future Gaia releases to establish whether those potential cluster-member stars are still dynamically bound to the clusters.Conclusions. The Gaia DR1 provides the means to examine open clusters far beyond their more easily visible cores, and can provide membership assessments based on proper motions and parallaxes. A combined HR diagram shows the same features as observed before using the HIPPARCOS data, with clearly increased luminosities for older A and F dwarfs
The (non-)variability of magnetic chemically peculiar candidates in the Large Magellanic Cloud
Context. The galactic magnetic chemically peculiar (mCP) stars of the upper main sequence are well known as periodic spectral
and light variables. The observed variability is obviously caused by the uneven distribution of overabundant chemical elements on
the surfaces of rigidly rotating stars. The mechanism causing the clustering of some chemical elements into disparate structures on
mCP stars has not been fully understood up to now. The observations of light changes of mCP candidates recently revealed in the
nearby Large Magellanic Cloud (LMC) should provide us with information about their rotational periods and about the distribution
of optically active elements on mCP stars born in other galaxies.
Aims. We queried for photometry at the Optical Gravitational Lensing Experiment (OGLE)-III survey of published mCP candidates
selected because of the presence of the characteristic λ5200 Å flux depression. In total, the intersection of both sources resulted in
twelve stars. For these objects and two control stars, we searched for a periodic variability.
Methods. We performed our own and standard periodogram time series analyses of all available data. The final results are, amongst
others, the frequency of the maximum peak and the bootstrap probability of its reality.
Results. We detected that only two mCP candidates, 190.1 1581 and 190.1 15527, may show some weak rotationally modulated light
variations with periods of 1.23 and 0.49 days; however, the 49% and 32% probabilities of their reality are not very satisfying. The
variability of the other 10 mCP candidates is too low to be detectable by their V and I OGLE photometry.
Conclusions. The relatively low amplitude variability of the studied LMC mCP candidates sample can be explained by the absence
of photometric spots of overabundant optically active chemical elements. The unexpected LMC mCPs behaviour is probably caused
by different conditions during the star formation in the LMC and the Galaxy.Web of Science556art. no. A1
Evaluating Discharge Regimes of Karst Aquifer
Karst springs are typical for abrupt changes of discharge immediately following recharge events. Monitored discharges of springs are used to determine quantitative variability over the period of time, showing their reliability as dependable water sources. Karst aquifers also exhibit (at least) dual ground-water flow regimes, that is, fast (conduit-dominated) flow and slow (diffuse) flow. This is something that can be observed in nature as the fast change of water amount outflowing from the groundwater source, or described by rapidly responding hydrographs, recording water levels or discharges. Selection of proper investigative techniques characterizing discharge regime properties of a karst aquifer is therefore important in order to identify possible theoretical background models describing this behaviour. On this basis, we can also find a particular method of hydrograph separation into flow components linked to the fast-flow regime, slow-flow regime, or intermediate regimes as well. With this point in mind, several quantitative methods that might be particularly useful in hydrograph analysis of water outlets from the karst aquifer system are briefly discussed here