Publications (27)32.11 Total impact
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Article: Discovery of Scuti pulsation in the Herbig Ae star VV Serpentis
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ABSTRACT: Context. The study of pulsation in pre-main-sequence intermediate mass stars represents an important tool for deriving information on the stellar parameters and structure, as well as for testing the validity of current theoretical models. The interest in this class of variable stars has significantly increased during the last decade and about 30 members are presently known in the literature. Aims. A new observational study of the Herbig Ae star VV Ser has been performed to detect and accurately measure pulsation frequencies in the δ Scuti range, thus enlarging the sample of known pulsators and contributing to the empirical definition of the pre-main-sequence instability strip. As it belongs to the continuous field of view of the asteroseismological satellite COROT, this study also aims at characterizing the properties of VV Ser as a potential "COROT additional program" candidate. Methods. CCD time series photometry in the Johnson V filter has been obtained for three consecutive years. The resulting light curves have been subject to detailed frequency analysis and the derived frequencies have been compared to model predictions. Results. Seven pulsation frequencies have been measured on the basis of the best data set obtained in 2004, ranging from ∼31 to ∼118 µHz, with an accuracy of the order of 0.5 µHz. The comparison with an extensive set of asteroseismological models shows that all the observed periodicities can be reproduced if the stellar mass is close to 4 M . Conversely, the measured frequencies can be associated with p modes only if the effective temperature is significantly lower than that obtained from the spectral type conversion. Conclusions. The present results seem to suggest that more accurate spectral type determination is necessary to discriminate the best-fit model solution. In any case, the stellar mass of VV Ser is close to the upper mass limit (∼4 M) for this class of pulsators.Astronomy and Astrophysics 01/2007; 462:1023-1030. · 4.59 Impact Factor -
Article: A multisite photometric campaign on the Pre-Main-Sequence delta Scuti pulsator IP Per
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ABSTRACT: We present the results of a photometric multisite campaign on the $\delta$ Scuti Pre-Main-Sequence star IP Per. Nine telescopes have been involved in the observations, with a total of about 190 hours of observations over 38 nights. Present data confirms the multiperiodic nature of this star and leads to the identification of at least nine pulsational frequencies. Comparison with the predictions of linear non-adiabatic radial pulsation models allowed us to identify only five of the nine observed frequencies, and to constrain the position of IP Per in the HR diagram. The latter is in good agreement with the empirical determination of the stellar parameters obtained by Miroshnichenko et al. (2001). An initial interpretation of the observed frequencies using the Aarhus non-radial pulsation code suggests that three frequencies could be associated with non-radial ($l$=2) modes. Finally, we present new evolutionary and pulsation models at lower metallicity (Z=0.008) to take into account the possibility that IP Per is metal deficient, as indicated by Miroshnichenko et al. (2001). Comment: 10 pages 11 figures, accepted for publication on A&A12/2005; -
Article: The GSC-II-based survey of ancient cool white dwarfs I. The sample of spectroscopically confirmed WDs
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ABSTRACT: The GSC-II white dwarf survey was designed to identify faint and high proper motion objects, which we used to define a new and independent sample of cool white dwarfs. With this survey we aim to derive new constraints on the halo white dwarf space density. Also, these data can provide information on the age of thick disk and halo through the analysis of the luminosity function. On the basis of astrometric and photometric parameters, we selected candidates with mu > 0.28 as/yr and R_F > 16 in an area of 1150 square degrees. Then, we separated white dwarfs from late type dwarfs and subdwarfs by means of the reduced proper motion diagram. Finally, spectroscopic follow-up observations were carried out to confirm the white dwarf nature of the selected candidates. We found 41 white dwarfs of which 24 are new discoveries. Here we present the full sample and for each object provide positions, absolute proper motions, photometry, and spectroscopy. Comment: 14 pages, 7 figures, submitted to A&A10/2005; -
Article: The GSC2 Survey of Cool White Dwarfs
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ABSTRACT: Microlensing experiments (Alcock et al. 2000) have suggested that a significant part of the dark halo of the Milky Way could be composed of matter in the form of massive compact halo objects (MACHOs). Cool ancient white dwarfs (WDs) are the natural candidates. Here we present a new survey of halo WDs and evaluate the local space density using an accurate kinematic analysis. A comparison to a re-derivation of the Oppenheimer et al. (2001) results is also provided. The local space density estimated from the two independent samples is only about ˜ 10-5Msunpc-3 corresponding to 0.1%-0.2% of the local dark matter.06/2005; 334:135. -
Article: On the PMS Star HBC 498 and Its Associated Nebulous Stars (Research Note)
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ABSTRACT: We present recent spectroscopic (resolution 1750 at = 6708 Ä) and photometric (ubvy-B) observations of the singular weak-line T Tauri star HBC 498 (= DL Ori/G1) still associated with its parental dust cloud and of at least four other bright neighboring stars associated with reflection nebulosities that together form a tight trapezium in L1641 in Orion. Most of them have the youth signature of Li I in their spectra. We find that two of the objects are ¿classic" T Tauri and another two are ¿weak-line" T Tauri stars. From the data and their comparison with models of pre-main sequence stellar evolution we find that they are coeval, and that they have masses of the order of 1 Mo and ages of the order of their gravitational contraction times or less (= 1 - 5 x 105 yr). Finally, we briefly discuss their evolutionary state and its repercussions.Revista Mexicana de Astronomía y Astrofísica. 01/2005; -
Chapter: Cool Halo White Dwarfs from GSCII
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ABSTRACT: Microlensing experiments have suggested that a significant part of the dark halo of the Milky Way could be composed of matter in form of massive compact halo objects (MACHOs). Cool ancient white dwarfs (WDs) are the natural candidates. Here we present a new survey of halo WDs and evaluate the local space density using a new method for the membership selection based on the kinematic properties of the stars. A comparison to a revaluation of the Oppenheimer et al. (2001) result is also provided. The local space density estimated from the two independent samples is about ∼ 10 5M⊙pc 3 and is consistent with the canonical local mass density of the halo WDs.12/2004: pages 31-39; -
Article: Observations of new candidate PMS $\delta$ Scuti stars
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ABSTRACT: Among variable stars, $\delta$ Scuti are particularly interesting since they can pulsate in different evolutionary phases and are characterized by both radial and nonradial pulsation modes. Even if the majority of $\delta$ Scuti stars are in their Main Sequence (MS) or post-MS phases, there are growing evidences in the recent years about the presence of $\delta$ Scuti variables among stars which are still in their pre-MS phase (the so-called Herbig Ae-stars). Due to the small number of the known class members, the boundaries of Instability Strip are not well constrained. In this context, we present the preliminary results of a long term project to define these limits by enlarging the sample of PMS $\delta$ Scuti stars.Proceedings of the International Astronomical Union 06/2004; 2004:812 - 817. -
Article: Multisite observations of the pre–Main-Sequence $\delta$ Scuti star IP Per
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ABSTRACT: We present preliminary results of a photometric multisite campaign on the $\delta$ Scuti-type Pre–Main-Sequence star IP Per. Nine telescopes have been involved in the observations, with a total of about 173 hour of observations over around 40 nights. Our current data permitted us to confirm the multiperiodic nature of this star and to determine at least 9 pulsational frequencies. A preliminary nonradial theoretical analysis seems to show that the star pulsates in a mixture of $l=0, 1, 2$ modes.Proceedings of the International Astronomical Union 06/2004; 2004:799 - 803. -
Article: Multisite observations of the PMS delta Scuti star V351 Ori
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ABSTRACT: We present the results of multisite observations spanning two years on the pre--main-sequence (PMS) star V351 Ori. A total of around 180 hours of observations over 29 nights have been collected, allowing us to measure five different periodicities, most likely related to the delta Scuti variability of V351 Ori. Comparison with the predictions of linear nonadiabatic radial pulsation models put stringent constraints on the stellar parameters and indicate that the distance to V351 Ori is intermediate between the lower limit measured by Hipparcos (210 pc) and that of the Orion Nebula (450 pc). However, radial pulsation models are unable to reproduce all of the observed frequencies with a single choice of (M, L, and Te), suggesting the presence of additional nonradial modes.Astronomy and Astrophysics 07/2003; 408:1047-1055. · 4.59 Impact Factor -
Article: Detection of delta Scuti-like pulsation in H254, a pre-main sequence F-type star in IC 348
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ABSTRACT: We present time series observations of intermediate mass PMS stars belonging to the young star cluster IC 348. The new data reveal that a young member of the cluster, H254, undergoes periodic light variations with delta Scuti-like characteristics. This occurrence provides an unambiguous evidence confirming the prediction that intermediate-mass pre-main sequence (PMS) stars should experience this transient instability during their approach to the main-sequence. On the basis of the measured frequency f=7.406 c/d, we are able to constrain the intrinsic stellar parameters of H254 by means of linear, non adiabatic, radial pulsation models. The range of the resulting luminosity and effective temperature permitted by the models is narrower than the observational values. In particular, the pulsation analysis allows to derive an independent estimate of the distance to IC 348 of about 320 pc. Further observations could either confirm the monoperiodic nature of H254 or reveal the presence of other frequencies. Comment: 7 pages, including 7 postscript figures, accepted for publication on A&A06/2002; -
Article: Pulsation in two Herbig Ae stars: HD 35929 and V351 Ori
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ABSTRACT: New photometric observations of seven intermediate mass pre-main sequence delta Scuti candidates are presented. The periods and pulsation modes are derived for two of these stars, namely HD 35929 and V351 Ori. The comparison between observations and nonlinear pulsational models allows us to provide some initial constraints on their mass and evolutionary state. As an illustration we discuss the use of periods to identify the mode of pulsation in these two stars and to have an independent estimate of their distances.03/2000; -
Article: On the nature of the ROSAT X-ray selected weak-line T Tauri stars in Orion
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ABSTRACT: We analyse the nature of the ROSAT all-sky survey (RASS) X-ray sources in the direction of Orion identified with stars showing the Li I lambdambda \:6708 Angstroms line strong in absorption and hence classified as weak-line T Tauri stars (WTTS) in a previous study. The stars are found to be widely spread throughout the entire studied area of ~ 450 squared degrees. We discuss the broad-band UBVRIKC JHKL and narrow-band uvby-beta photometry as well as the spectroscopy of these stars. From the broad-band photometry and spectroscopy we derive the stellar parameters assuming that all stars are located at 460 pc and are physically associated with the Orion star forming region (SFR). By comparison with theoretical pre-main sequence (PMS) evolutionary tracks, all stars can be classified as WTTS with masses raging from 0.8Msun to about 3.4Msun and ages from 2x10(5) yr to 7x10(6) yr. We do not find any correlation between the spatial distribution and age or any other stellar parameter if the above distance for all the stars is assumed. We do find, however, that the stars with higher Li I (lambdambda 6708 Angstroms) line strength tend to concentrate toward the molecular clouds. From the analysis of the uvby-beta photometric data we find that part of a subsample of the RASS lithium stars are foreground young stars not associated with Orion. We conclude that the sample of lithium RASS stars in Orion is an admixture of different populations of stars located at different distances, namely: true Orion WTTS and a population of foreground and yet young stars. The latter could be associated with the Gould Belt or may be pleiades-age stars.Astronomy and Astrophysics 01/1998; 330:1017-1028. · 4.59 Impact Factor -
Article: A study of the Chamaeleon star-forming region from the ROSAT all-sky survey. III. High resolution spectroscopic study
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ABSTRACT: We present the results of a high-resolution spectroscopic study on some 70 stars, discovered recently on the basis of the ROSAT all-sky survey spread over a wide area in the Chamaeleon star forming region and classified as new weak-line T Tauri stars. We refine the previous spectral type classification, based on low-resolution spectra, and characterize each star in the sample according to the Hα line profile. We use the strength of Li I 6708, compared to Pleiades stars of the same spectral type, as a youth discriminator in order to recognize bona-fide pre-main sequence stars. According to the adopted ``lithium criterion'', more than 50% of the stars in our sample are confirmed to be truly young, PMS stars (most having age less than 5x 10(6) yr), while the remaining part seems mostly composed by active, young, foreground main-sequence stars (possibly Pleiades-like), which contaminate the original sample. We confirm the existence of some very young stars far from the main Chamaeleon clouds, while we do not find clear evidence for the presence of post-T Tauri stars in our sample. We find that 5 stars in the sample are spectroscopic binaries and 1 is a spectroscopic triple system. We derive radial and rotational velocities for all the stars in sample and analyse their distributions for different spectral type intervals. The radial velocity distribution shows a clear peak at about 15 km s(-1) , which coincides with the radial velocity of stars and gas in the Cha I cloud. However, the velocity dispersion of the weak-line T Tauri stars appears much broader and, possibly, a second peak is present around 16-18 km s(-1) . A clear segregation in radial velocity is observed between the strong-lithium and the weak-lithium stars, with the former showing radial velocities which, in most cases, fall in the interval 12<RV [km/s] < 18, consistent with the radial velocity peak observed for the Cha I dark cloud, and the latter having somewhat different and more widely spread radial velocities. Some strong-lithium stars having radial velocities outside the aforementioned interval might be unrecognized spectroscopic binaries, but they may also be considered good candidates to be run-away T Tauri stars. The vsin i distribution of the confirmed WTTS is found to resemble very closely that of other pre-ROSAT PMS stars. We conclude that the whole star forming region has a common origin, possibly related to the impact of a high-velocity cloud with the galactic plane. Based on observations collected at the European Southern Observatory at La Silla, ChileAstronomy and Astrophysics 11/1997; 328:187-202. · 4.59 Impact Factor -
Article: New weak-line T Tauri stars in Orion from the ROSAT all-sky survey.
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ABSTRACT: We present results of the spectroscopic and photometric follow-up observations of the ROSAT all-sky survey in the direction of the Orion cloud complex. The main goal of these observations is the search for X-ray emitting pre-main sequence stars. 820 X-ray sources were detected with high confidence in about 450 square degrees. The mean density of X-ray sources in this region is a factor of about two higher than that of the whole RASS. 5% of the RASS sources in this region are identified with previously known and likely pre-main sequence stars. We have investigated spectroscopically 181 new RASS sources widely distributed over the entire cloud complex. On the basis of the presence of strong Li I λ6707 absorption, spectral type later than F0 and chromospheric emission, 112 new weak-line T Tauri stars could be found. We present coordinates, X-ray count-rates and finding charts of the new PMS. Optical UBV(RI)_KC_, near-infrared JHKLM and uvby-β photometry for the new WTTS is also provided. In addition 24 dKe-dMe stars were also found on the basis of the RASS data.Astronomy and Astrophysics Supplement Series 09/1996; 119:7-24. -
Article: Hα Interferometric Optical and Near Infrared Photometric Studies of Star Forming Regions - Part Two - the NGC7380E / SH:2-142 / NGC7380 Complex
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ABSTRACT: Here we report the results of an uvby-β and JHKLM photometric study of 25 stars in the field direction of the open cluster NGC 7380, and of an Hα interferometric/photographic direct imagery study of the HII and its associated molecular cloud regions Sh2-142 and NGC 7380 E, respectively. The photometric study yielded the following: a) Teff and log g estimates of the program stars, (b) a normal reddening law and uniform extinction towards the region with Aυ/E(B - V) = 3.1±0.1 and Aυ = 2m.0±0m.1, respectively, (c) the result that 16 of the 25 stars observed photometrically are cluster members and the remaining 9 are field stars, (d) and that only 4 of the 16 cluster members studied here have spectral types later than B 1, and (e) similar distance estimates to NGC 7380 by three distinct methods with d = 3.2±0.2 Kpc as its recommended value. From the Hα interferometric imagery of Sh2-142 we find the following: (f) the distribution of the 1100 individual radial velocity points conforming the velocity field of the HII region follow a Gaussian distribution with a mean (LSR) radial velocity of -40.3±0.2 km s-1 and a velocity dispersion of 1σ = 5.1 km s-1, (g) a kinematic distance estimate of dk = 3.3±0.2 Kpc, and (h) a lack of confirmation of the velocity gradient of about 30 km s-1 in the east-west direction of the nebula and the velocity dispersion of 12.5 km s-1 observed with similar techniques by others. The photographic imagery of Sh2-142 in the Hα λ6563Å, [NII]λ6584Å and [SII]λλ6717/6731Å lines reveal (i) the peculiar structure of the nebula composed by several bright arcuated clouds, rims and Hα knots interlaced with local dust lanes, (j) a resemblance between the Hα image and thermal 21 cm radio continuum map in form and size, and (k) the bright knot of about 0.4 pc in diameter is being excited at least partially by the O8V star LSIII 57°89 (nr. 16 here). The current model for the HII/HI/H2 cloud complex is briefly discussed in the context of our observations.Astronomy and Astrophysics 02/1994; 283:963. · 4.59 Impact Factor -
Article: T-Chamaeleontis - a Weak-Line Yy-Orionis Star
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ABSTRACT: We present new low and high resolution spectroscopy and optical photometry of the star T Cha. From the low resolution spectroscopy we classify T Cha as a G8 type star. We also identify T Cha as the optical counterpart of the ROSAT all- sky survey X-ray source RX J1 157.2-7921. On the basis of the physical association with the Chamaeleon cloud complex, the erratic photometric variability and colour behaviour, the presence of Hα emission and of strong Li λ 6708 absorption line, as well as its position on the HR diagram, we establish unambiguously the low-mass PMS nature of the star. These characteristics, combined with an observed equivalent width of the Hot emission less than 10 Å, suggest that the star can be classified as a weak-line T Tauri star. However, the strong IR excess of the star, resembling more closely those of classical T Tauri stars, clearly indicates the presence of a circumstellar disk. From spectra obtained at different epochs we find that strong variability is present in the Hot line, with the profile changing from pure emission to an inverse P Cygni on a time scale of one day or less. Actually, T Cha is the first case in which an inverse P Cygni profile is observed at Hα. Hence, here we suggest that T Cha can be considered a case of "weak-line" YY Orionis star.Astronomy and Astrophysics 04/1993; 272:225. · 4.59 Impact Factor -
Article: UBV(RI)C photometric monitoring of Orion population stars in the Southern Hemisphere
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ABSTRACT: UBV(RI)C photometry on a sample of southern T Tauri stars, selected from the Herbig and Bell (1988) catalog, is reported and four new candidates (AS 205, Wa Oph/2, Wa Oph/3, and Wa CrA/2) for rotational modulation are singled out from the sample. The color trend for the stars showing variability is considered. Typically, colors become redder when the star is fainter, complying with the general behavior observed in T Tauri stars. This can be interpreted as due to the (variable) presence of surface inhomogeneities (either dark or bright) or, in some cases, it could also be consistent with the hypothesis of variable circumstellar extinction. Estimates of gross spot properties (e.g., temperature and filling factor) are derived for the four stars showing possible periodicity, from the amplitudes of the variations observed at different wavelengths, by use of a simple blackbody model.Astronomy and Astrophysics Supplement Series 07/1992; 94:273-290. -
Article: A high-resolution spectroscopic study of southern T Tauri stars
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ABSTRACT: We present the results of a high resolution spectroscopic study of five southern T Tauri stars: Sz 19, T Cha, Sz 68, Wa CrA/2, and TW Hya. The spectra cover a wavelength range of about 50 A centered on the Na I D lines. Simultaneous UBV(RI)c photometry was also obtained. Spectral types and projected rotational velocities have been derived by comparison with reference stars. Heliocentric radial velocities of the stars and their parent clouds have been determined. Night-to-night variability in the line profiles of T Cha and Sz 68 was detected. The Na I D profile of Sz 68 displays at least three systems of narrow blue-shifted absorption components in the Na I D lines with only the highest velocity one showing night-to-night variability in both intensity and width. This suggests a complex and unsteady circumstellar structure.Astronomy and Astrophysics 02/1992; 256:525-532. · 4.59 Impact Factor -
Article: The low-mass pre-main sequence population among the ROSAT X-ray selected stars in the Chamaleon cloud complex
Memorie della Societa Astronomica Italiana. 68:1057. -
Article: Different populations among the lithium-rich stars discovered from the ROSAT all-sky survey in Orion.
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