arXiv subjects

V. Costa

Publications and source records attributed to V. Costa.

13 recordsLinked to original sources

arXiv2010

Improvements in the X-ray luminosity function and constraints on the Cosmological parameters from X-ray luminous clusters

We show how to improve constraints on Ω_m, σ_8, and the dark-energy equation-of-state parameter, w, obtained by Mantz et al. (2008) from measurements of the X-ray luminosity function of galaxy clusters, namely MACS, the local BCS and the REFLEX galaxy cluster samples with luminosities L> 3 \times 10^{44} erg/s in the 0.1--2.4 keV band. To this aim, we use Tinker et al. (2008) mass function instead of Jenkins et al. (2001) and the M-L relationship obtained from Del Popolo (2002) and Del Popolo et al. (2005). Using the same methods and priors of Mantz et al. (2008), we find, for a Λ$CDM universe, Ω_m=0.28^{+0.05}_{-0.04} and σ_8=0.78^{+0.04}_{-0.05}$ while the result of Mantz et al. (2008) gives less tight constraints $Ω_m=0.28^{+0.11}_{-0.07}$ and σ_8=0.78^{+0.11}_{-0.13}. In the case of a wCDM model, we find Ω_m=0.27^{+0.07}_{-0.06}, $σ_8=0.81^{+0.05}_{-0.06}$ and $w=-1.3^{+0.3}_{-0.4}$, while in Mantz et al. (2008) they are again less tight Ω_m=0.24^{+0.15}_{-0.07}, σ_8=0.85^{+0.13}_{-0.20} and w=-1.4^{+0.4}_{-0.7}. Combining the XLF analysis with the f_{gas}+CMB+SNIa data set results in the constraint Ω_m=0.269 \pm 0.012, σ_8=0.81 \pm 0.021 and w=-1.02 \pm 0.04, to be compared with Mantz et al. (2008), Ω_m=0.269 \pm 0.016, σ_8=0.82 \pm 0.03 and w=-1.02 \pm 0.06. The tightness of the last constraints obtained by Mantz et al. (2008), are fundamentally due to the tightness of the $f_{gas}$+CMB+SNIa constraints and not to their XLF analysis. Our findings, consistent with w=-1, lend additional support to the cosmological-constant model.

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arXiv2008

The gamma Dor CoRoT target HD49434. I-Results from the ground-based campaign

Context: We present the results of an extensive ground-based photometric and spectroscopic campaign on the gamma Dor CoRoT target HD49434. This campaign was preparatory to the CoRoT satellite observations, which took place from October 2007 to March 2008. Results: The frequency analysis clearly shows the presence of four frequencies in the 0.2-1.7 c/d interval, as well as six frequencies in the 5-12 c/d domain. The low frequencies are typical for gamma Dor variables while the high frequencies are common for delta Sct pulsators. We propose the frequency 2.666 c/d as a possible rotational frequency. All modes, for which an identification was possible, seem to be high-degree modes (3 <= l <= 8). We did not find evidence for a possible binary nature of HD49434. The element abundances we derived are consistent with the values obtained in previous analyses. Conclusions: We classify the gamma Dor star HD49434 as a hybrid pulsator, which pulsates simultaneously in p- and g-modes. This finding makes HD49434 an extremely interesting target for asteroseismic modelling. Observations: Based on observations made with the 2.2m ESO/MPI telescope at the La Silla Observatory under the ESO Large Programme: LP178.D-0361. Also based on observations obtained at Observatorio de Sierra Nevada (Spain), at the Centro Astronomico Hispano Aleman at Calar Alto (Spain), at Observatorio Astronomico Nacional San Pedro Martir (Mexico), at the Piszkesteto Mountain Station of Konkoly Observatory (Hungary), at Observatoire de Haute Provence (France) and at Mount John University Observatory (New Zealand).

astro-ph↗

arXiv2008

Sources of uncertainties in the s-process in massive stars: convection and reaction rates

Current models of s-nucleosynthesis in massive stars ($M\sim15 M_{\odot}$ to $\sim 30 M_{\odot}$) are able to reproduce some main features of the abundance distributions of heavy isotopes in the solar system, at least in the $A\sim 60-90$ mass range. The efficiency of the process and the above specified mass range for the s-nuclei are still heavily uncertain due to both nuclear reaction rates and stellar models uncertainties. A series of s-process simulations with stellar models in the $15-30 M_{\odot}$ (mass at ZAMS) and metallicity $Z=0.02$ mass have been performed to analyse the impact of the overshooting model used on the s-process yields. As in a previous exploratory work performed with stellar models having $M_{ZAMS}=25 M_{\odot}$ and $Z=0.02$, enhancements factors in the range 2-5 are found in the final s-process efficiency when overshooting is inserted in the models.

astro-ph↗

arXiv2007

Multiperiodicity in the newly discovered mid-late Be star V2104 Cygni

We obtained the first long, homogenous time-series of V2104Cyg, consisting of 679 datapoints, with the uvbybeta photometers of Sierra Nevada and San Pedro Martir Observatories with the aim to detect and subsequently interpret the intrinsic frequencies of this previously unstudied variable star, which turned out to be a Be star. We try to figure out its place among the variable B stars on the upper Main Sequence. In order to obtain additional information on physical parameters we collected a few spectra with the ELODIE and FIES instruments. We searched for frequencies in the uvby passbands using 2 different frequency analysis methods and used the S/N>4 criterion to select the significant periodicities. We obtained an estimate of the physical parameters of the underlying B star of spectral type between B5 and B7, by correcting for the presence of a circumstellar disk, using a formalism based on the strenght of the Halpha line emission. We detected 3 independent frequencies with amplitudes below 0.01mag, f1 = 4.7126 c/d, f2 = 2.2342 c/d and f3 = 4.671 c/d, and discovered that V2104Cyg is a Be star. The fast rotation (vsini=290+/-10 km/s, and 27<i<45) hampered the investigation of the associated pulsational parameters l. Nevertheless, the most plausible explanation for the observed variability of this mid-late type Be star is a non-radial pulsation model. This paper is based on observations obtained at the Observatorio Astronomico Nacional San Pedro Martir (Mexico), Observatorio de Sierra Nevada (Spain), Observatoire de Haute Provence (France), and on observations made with the Nordic Optical Telescope, Observatorio Roque de los Muchachos, La Palma, Spain.

astro-ph↗

arXiv2006

Asteroseismology with the WIRE satellite. I. Combining Ground- and Space-based Photometry of the Delta Scuti Star Epsilon Cephei

We have analysed ground-based multi-colour Stromgren photometry and single-filter photometry from the star tracker on the WIRE satellite of the delta scuti star Epsilon Cephei. The ground-based data set consists of 16 nights of data collected over 164 days, while the satellite data are nearly continuous coverage of the star during 14 days. The spectral window and noise level of the satellite data are superior to the ground-based data and this data set is used to locate the frequencies. However, we can use the ground-based data to improve the accuracy of the frequencies due to the much longer time baseline. We detect 26 oscillation frequencies in the WIRE data set, but only some of these can be seen clearly in the ground-based data. We have used the multi-colour ground-based photometry to determine amplitude and phase differences in the Stromgren b-y colour and the y filter in an attempt to identify the radial degree of the oscillation frequencies. We conclude that the accuracies of the amplitudes and phases are not sufficient to constrain theoretical models of Epsilon Cephei. We find no evidence for rotational splitting or the large separation among the frequencies detected in the WIRE data set. To be able to identify oscillation frequencies in delta scuti stars with the method we have applied, it is crucial to obtain more complete coverage from multi-site campaigns with a long time baseline and in multiple filters. This is important when planning photometric and spectroscopic ground-based support for future satellite missions like COROT and KEPLER.

astro-ph↗

arXiv2006

Discovery and application of Frequency Ratio Method to the new multiperiodic gamma Dor star HD 218427

gamma Dor-type oscillations have been discovered in the star HD218427 through simultaneous uvby photometric observations carried out in the year 2003. A few Hbeta-Crawford measurements were also collected for calibration purposes which place this star well-located inside the gamma Dor instability region. Deficiency in metal content, similar to other well-defined gamma Dor stars, is found in HD218427 and the possibility of a lambda Boo nature is discussed. Frequency analysis was carried out for different filters, the combined "vby" filter was also used and five frequencies were found as significant with periods ranging between 0.3 and 0.8 days. The recently developed Frequency Ratio Method is used in order to perform an identification of the excited modes. The results are consistent with an l=2 identification for all the modes and high radial quantum numbers (n~40) for the three main observed periodicities. The possibility of multiplet structures is also discussed. However, no consistency is found when the Time-Dependent Convection treatment is used for modes discrimination. The disagreement can be due to the large rotation velocity taking place in HD218427 and, consequently, the significant coupling between the modes.

astro-ph↗

arXiv2005

Detection of 75+ pulsation frequencies in the Delta Scuti star FG Vir

Extensive photometric multisite campaigns of the Delta Scuti variable FG Vir are presented. For the years 2003 and 2004, 926 hours of photometry at the millimag precision level were obtained. The combinations with earlier campaigns lead to excellent frequency resolution and high signal/noise. A multifrequency analysis yields 79 frequencies. This represents a new record for this type of star. The modes discovered earlier were confirmed. Pulsation occurs over a wide frequency band from 5.7 to 44.3 c/d with amplitudes of 0.2 mmag or larger. Within this wide band the frequencies are not distributed at random, but tend to cluster in groups. A similar feature is seen in the power spectrum of the residuals after 79 frequencies are prewhitened. This indicates that many additional modes are excited. The interpretation is supported by a histogram of the photometric amplitudes, which shows an increase of modes with small amplitudes. The old question of the 'missing modes' may be answered now: the large number of detected frequencies as well as the large number of additional frequencies suggested by the power spectrum of the residuals con rms the theoretical prediction of a large number of excited modes. FG Vir shows a number of frequency combinations of the dominant mode at 12.7162 c/d (m = 0) with other modes of relatively high photometric amplitudes. The amplitudes of the frequency sums are higher than those of the differences. A second mode (20.2878 c/d) also shows combinations. This mode of azimuthal order m = -1 is coupled with two other modes of m = +1.

astro-ph↗

arXiv2005

Asteroseismology of the Beta Cephei star Nu Eridani: photometric observations and pulsational frequency analysis

We undertook a multisite photometric campaign for the Beta Cephei star Nu Eridani. More than 600 hours of differential photoelectric uvyV photometry were obtained with 11 telescopes during 148 clear nights. The frequency analysis of our measurements shows that the variability of Nu Eri can be decomposed into 23 sinusoidal components, eight of which correspond to independent pulsation frequencies between 5 - 8 c/d. Some of these are arranged in multiplets, which suggests rotational m-mode splitting of nonradial pulsation modes as the cause. If so, the rotation period of the star must be between 30 - 60 d. One of the signals in the light curves of Nu Eri has a very low frequency of 0.432 c/d. It can be a high-order combination frequency or, more likely, an independent pulsation mode. In the latter case Nu Eri would be both a Beta Cephei star and a slowly pulsating B (SPB) star. The photometric amplitudes of the individual pulsation modes of Nu Eri appear to have increased by about 20 per cent over the last 40 years. So do the amplitudes of the dominant combination frequencies of the star. Among the latter, we only could identify sum frequencies with certainty, not difference frequencies, which suggests that neither light-curve distortion in its simplest form nor resonant mode coupling are their single cause. One of our comparison stars, Mu Eridani, turned out to be variable with a dominant time scale of 1.62 d. We believe that it is either an SPB star just leaving its instability strip or that its variations are of rotational origin.

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arXiv2004

GAUDI: a preparatory archive for the COROT mission

The GAUDI database (Ground-based Asteroseismology Uniform Database Interface, http://sdc.laeff.esa.es/gaudi/) is a preparatory archive for the COROT (COnvection, ROtation and planetary Transits, http://www.astrsp-mrs.fr/projets/corot/) mission developed at LAEFF (Laboratory for Space Astrophysics and Theoretical Physics, http://www.laeff.esa.es). Its intention is to make the ground-based observations obtained in the preparation of the asteroseismology programme available in a simple and efficient way. It contains spectroscopic and photometric data together with inferred physical parameters for more than 1500 objects gathered since January 1998 in 6 years of observational campaigns. In this paper, the main functionalities and characteristics of the system are described. The observations have been collected at ESO-La Silla, Telescopio Nazionale Galileo, Observatoire de Haute-Provence, South African Astronomical Observatory, Tautenberg Observatory and Sierra Nevada Observatory.

astro-ph↗

arXiv2004

The Delta Scuti star FG Vir. V. The 2002 photometric multisite campaign

A high-accuracy multisite campaign was carried out from 2002 January to May with a photometric coverage of 398 hours at five observatories. The concentration on a few selected sites gives better consistency and accuracy than collecting smaller amounts from a larger number of sites. 23 frequencies were detected with a high statistical significance. 6 of these are new. The 17 frequencies found in common with the 1992-1995 data are the modes with highest amplitudes. This indicates that the pulsation spectrum of FG Vir is relatively stable over the ten-year period. Two frequencies have variable amplitudes and phases from year to year as well as during 2002. These were both found to be double modes with close frequencies. For the mode at 12.15 c/d this leads to an apparent modulation with a time scale of about 129d. The close frequencies at 12.15 c/d are composed of a radial and a nonradial mode, suggesting a similarity with the Blazhko Effect seen in RR Lyrae stars.

astro-ph↗

arXiv2002

Coordinated Multiwavelength Observations of V410Tau

In November 2001 we undertook a coordinated observing campaign to study the connection between X-ray and optical variability in the weak-line T Tauri star V410Tau. The observing plan included three 15 ksec observations with Chandra using the Advanced CCD Imaging Spectrometer for Spectroscopy scheduled for different phases of the known 1.87 d starspot cycle. Photometric and spectroscopic monitoring of V410Tau involving telescopes on three different continents was scheduled simultaneously with the Chandra exposures.

astro-ph↗

arXiv2000

News from the p-process: is the s-process a troublemaker?

The most detailed calculations of the p-process call for its development in the O/Ne layers of Type II supernovae. In spite of their overall success in reproducing the solar system content of p-nuclides, they suggest a significant underproduction of the light Mo and Ru isotopes. On grounds of a model for the explosion of a 25 solar mass star with solar metallicity, we demonstrate that this failure might just be related to the uncertainties left in the rate of the 22Ne(alpha,n)25Mg neutron producing reaction. The latter indeed has a direct impact on the distribution of the s-process seeds for the p-process.

astro-ph↗

arXiv2000

The synthesis of the light Mo and Ru isotopes: how now, no need for an exotic solution ?

The most detailed calculations of the p-process call for its development in the O/Ne layers of Type II supernovae. In spite of their overall success in reproducing the solar system content of p-nuclides, they suggest a significant underproduction of the light Mo and Ru isotopes. On grounds of a model for the explosion of a 25 solar mass star with solar metallicity, we demonstrate that this failure might just be related to the uncertainties left in the rate of the 22Ne(alpha,n)25Mg neutron producing reaction. The latter indeed have a direct impact on the distribution of the s-process seeds for the p-process.

astro-ph↗