Search

Aims. We aim at determining the rotational periods and the starspot properties in very young low-mass stars belonging to the Ori OB1c star forming region, contributing to the study of the angular momentum and magnetic activity evolution in these objects.

Aims. In this article we describe the search for white dwarfs (WDs) in the multi-band photometric data of the Capodimonte deep field survey.

Aims.We present the results of an optical multi-band survey for low-mass pre-main sequence (PMS) stars and young brown dwarfs (BDs) in the Chamaeleon II (Cha II) dark cloud. This survey constitutes the complementary optical data to the c2d Spitzer Legacy survey in Cha II.

Context.A severe problem for research in star-formation is that the masses of young stars are almost always estimated from evolutionary tracks alone. Since the tracks published by different groups differ, it is often only possible to give a rough estimate of the masses of young stars. It is thus crucial to test and calibrate the tracks. Up to now, only a few tests of the tracks could be carried out. However, it is now possible with the VLTI to set constrains on the tracks by determining the masses of many young binary stars precisely.

Aims.Our goal is to recover the Galactic halo spatial density by means of field stars. To this aim, we have applied a new technique to the Capodimonte Deep Field (OACDF, Alcalá et al. 2004), as a pilot study in view of the VLT Survey Telescope (VST) stellar projects. Considering this unique chance to collect deep and wide-field photometry with the VST, our method may represent a useful tool towards definitive mapping of the Galactic halo.

Context.The evolution of the angular momentum in young low-mass stars is still a debated issue. The stars presented here were discovered as X-ray sources in the ROSAT All-Sky Survey (RASS) of the Orion complex and subsequently optically identified thanks to both low and high resolution spectroscopy.

In this letter we characterize the candidates for young stars and brown dwarfs detected in a survey with ISOCAM in the Chamaeleon II dark cloud. Photometric data coming from a wide-field optical imaging survey, combined with IR measurements from the 2MASS catalogue and from the Spitzersatellite, allow us to study the nature of the candidates. Using intermediate-band filters we can provide a first estimate of the temperatures for objects cooler than about 3800 K. On the basis of spectroscopic and photometric data, we confirm that ISO-CHA II 13 is a young substellar object with a mass of about $0.05~M_{\odot}$. It is thus the first object in Cha II whose substellar nature has been spectroscopically confirmed. By having a temperature of about 2880 K and displaying a substantial infrared excess, it joins the list of young brown dwarfs observed to have a surrounding disk.

We present results of an investigation of the X-ray properties, age distribution, and kinematical characteristics of a high-galactic latitude sample of late-type field stars selected from the ROSAT All-Sky Survey (RASS). The sample comprises 254 RASS sources with optical counterparts of spectral types F to M distributed over six study areas located at $|b| \ga 20\degr$, and ${\rm Dec} \ge -9\degr$. A detailed study was carried out for the subsample of ~200 G, K, and M stars. Lithium abundances were determined for 179 G-M stars. Radial velocities were measured for most of the 141 G and K type stars of the sample. Combined with proper motions these data were used to study the age distribution and the kinematical properties of the sample. Based on the lithium abundances half of the G-K stars were found to be younger than the Hyades (660 Myr). About 25% are comparable in age to the Pleiades (100 Myr). A small subsample of 10 stars is younger than the Pleiades. They are therefore most likely pre-main sequence stars. Kinematically the PMS and Pleiades-type stars appear to form a group with space velocities close to the Castor moving group but clearly distinct from the Local Association.

We present the Capodimonte Deep Field (OACDF), a deep field covering an area of 0.5 $\deg ^2$in the B, V, Roptical bands plus six medium-band filters in the wavelength range 773–913 nm. The field reaches the following limiting magnitudes: $B_{AB}\sim 25.3$, $V_{AB}\sim 24.8$and $R_{AB}\sim25.1$and contains ~50 000 extended sources in the magnitude range 18 $\le R_{AB} \le 25.0$. Hence, it is intermediate between deep pencil beamsurveys and very wide but shallow surveys. The main scientific goal of the OACDF is the identification and characterization of early-type field galaxies at different look-back times in order to study different scenarios of galaxy formation. Parallel goals include the search for groups and clusters of galaxies and the search for rare and peculiar objects (gravitational lenses, QSOs, halo White Dwarfs). In this paper we describe the OACDF data reduction, the methods adopted for the extraction of the photometric catalogs, the photometric calibration and the quality assessment of the catalogs by means of galaxy number counts, spectroscopic and photometric redshifts and star colors. We also present the first results of the search for galaxy overdensities. The depth of the OACDF and its relatively large spatial coverage with respect to pencil beam surveys make it a good tool for further studies of galaxy formation and evolution in the redshift range 0–1, as well as for stellar studies.

We report new photometric observations and a new determination of the fundamental stellar parameters for the low-mass pre-main sequence eclipsing system RX J0529.4+0041A based on high-precision, near-IR ($\it JHK$) differential photometry obtained using adaptive optics at the ESO-La Silla 3.6 m telescope, and $\it UBV(RI)_{\rm C}$CCD photometry performed with the OIG camera at TNG. The new photometric data, in combination with already published photoelectric photometry and solution of the radial velocity curve, yield a more precise determination of the absolute dimensions and masses as well as of other basic physical properties of the components.

We present the results of a multi-wavelength study of the star forming region in L1616. Our observations include ROSAT All-Sky Survey (RASS) and High Resolution Imager (HRI) X-ray observations, optical wide-field imaging and near-IR imaging data and optical long-slit and multi-object spectroscopic follow-up. 22 new low-mass pre-main sequence (PMS) stars are found to be distributed mainly to the East of the L1616 cometary cloud, in about a one-square-degree field. We find that the class-III infrared sources outnumber the class-II infrared sources by a factor of about three. The X-ray properties of the PMS stars in L1616 are quite similar to those of PMS stars detected in the Orion Nebula Cluster. The comparison of the position of the L1616 PMS stars in the HR diagram with theoretical PMS evolutionary tracks yields an average age of 1–2 Myr, with a very small age spread of about 1 Myr. Unlike the fossil star forming regions in Orion, L1616 appears to be a region of on-going star formation relatively far from the Orion A and B clouds. Given the small age spread, the spatial distribution of the PMS stars relative to the head of the cloud, as well as its cometary shape and high star formation efficiency, we conclude that the star formation in L1616 was most likely induced by a single event, the impact of the winds of the massive stars of the Orion OB association or a supernova explosion being the possible triggers. The Initial Mass Function (IMF) in L1616 is roughly consistent with that of the field in the mass range $0.3< M/M_\odot < 2.5$. Several faint objects, detected in our optical images, are good candidates for young Brown Dwarfs (BDs). We might expect the number of BDs in L1616 to be intermediate between Taurus and the Trapezium.

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 δScuti 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 ($M_\ast$, $L_\ast$, and Teff), suggesting the presence of additional nonradial modes.

We report on the identification of 22 ROSAT All-Sky Survey (RASS) X-ray sources distributed in the general direction of the Orion star-forming region. The X-ray sample contains sources from the ROSAT bright source catalogue and from previous detections. The optical identifications are based on intermediate-resolution spectroscopy and UBVJohnson photometry using a 1m-class telescope. The strengths of the Hα, $\ion{Na}{i}$D2and lithium lines for the stellar counterparts are evaluated applying the spectral subtraction technique, using templates of the same spectral type. Radial velocities of the optical counterparts are also reported. Thirteen of the optical counterparts show the lithium absorption line in their spectra and have radial velocities consistent with the Orion star forming region. Four of these objects can be classified as new bona-fide T Tauri stars.

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 δScuti-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 $f=7.406$d-1, 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.

We report high-resolution spectroscopic observations, as well as high-resolution near infrared (IR) imaging of six stars previously identified in a ROSAT pointed observation in the direction of the B-type star βCru, and classified as low-mass pre-main sequence (PMS) stars. Four of the stars are confirmed to be low-mass PMS stars, associated with the Lower Centaurus-Crux group, while the other two are unrelated to the Sco-Cen association. The confirmed PMS stars are most likely in their post-T Tauri evolutionary phase. Although future deep X-ray observations with high-resolution imagers might detect more new PMS stars, the possibility that the Crux PMS stars are part of a small aggregate, with βCrux itself approximately at the center, is rather unlikely, given the high velocity dispersion and the low spatial density of the confirmed PMS stars. Instead, these stars may be part of a moving group in a more disperse and numerous population of low-mass PMS stars, distributed in the Lower Centaurus-Crux subgroup. New PMS binaries and multiple systems were also discovered among the stars in the sample, namely a close visual pair and a hierarchical triple system in which one of the components is a double-lined spectroscopic binary (SB2). The detailed orbital solution is reported for the inner short-period ($P_{\rm orb} =$58.3 days) SB2. A preliminary orbital solution for the hierarchical triple system yields a systemic orbital period of about 4.6 years, which makes this object a very suitable target for follow-up observations with the Very-Large Telescope Interferometer (VLTI) in the coming years.

In this paper we present an updated compilation of the currently known pre-main sequence (PMS) spectroscopic binaries (SB) for which orbital elements have been determined. In particular we report our results for and discuss the case of the bona-fidePMS system RX J1603.9-3938, which has a circular orbit and a period of 7.56 days. This is the longest orbital period for a circular orbit found among the bona-fide PMS spectroscopic binaries so far, and we suggest that this system may be thus considered to now set the PMS circularization period. The longer period is compatible with the circularization periods already known for older binary populations such as the Hyades and Praesepe. When considered in the context of the circularization periods for other binary populations of different ages, the new PMS circularization period strongly supports the suggestion of Mathieu et al. ([CITE]) that a hybrid scenario (tidal circularization occurring on both PMS phase and on MS phase) could explain the observed circularization periods as a function of age. The apparently circular orbit observed in another PMS system, RX J1301.0-7654a, with an orbital period of nearly 13 days, may perhaps change this picture when the orbit is improved and the system is better understood.

We report the results of a high-resolution spectroscopic monitoring campaign on low-mass pre-main sequence spectroscopic binaries, discovered recently in the general direction of the Orion star-forming region, based on ROSAT all-sky survey X-ray observations. Also included in the present study are two binaries recognized in the course of optical follow-up observations of X-ray sources in a selected sky strip crossing the Orion SFR perpendicular to the galactic plane. Orbital elements for the six double-lined spectroscopic binaries are derived from the analysis of the radial velocities of the components. The orbital periods span from 3 to 47 days. In addition, through a matching of the binary composite spectrum with synthetic binary spectra, we estimate spectral types and luminosity ratios for the components and derive lithium abundances for individual binary components. Using the estimated stellar parameters combined with kinematical information and lithium abundance determinations, we examine the evolutionary status of the objects. We then use the minimum masses derived from the solution of the spectroscopic orbits for the systems of confirmed PMS nature to make comparisons with current theoretical pre-main sequence evolutionary tracks, and attempt to set constraints on some of the most frequently used models.

BS Ind (=HD 202947) is a young K0V star in the Tucana association. Photometric observations with the Hipparcos satellite show an eclipse-like light-curve with primary and secondary eclipse. The eclipsing binary has a period of 0.435338 days and a circular orbit. Our spectroscopic observations however show that the K0V primary is a single-line spectroscopic binary with a period of 3.3 years. The minimum mass of the invisible component is about 0.9 $M_\odot$ which means that the mass of the companion is about the same as that of the primary. The first inspection of our FEROS spectra with a resolution of 48 000, as well as a CES spectrum with a resolution of 220 000 shows no obvious companion. However, when the FEROS spectra are cross-correlated with an M-star, a secondary becomes visible as a broad peak in the cross-correlation function. The width and the position of this broad peak is variable on a short time. When phased to a period of 0.435338 days, the radial velocity variations of the broad peak show the characteristic sine-wave of a spectroscopic binary in a circular orbit. The best interpretation of this data is that the broad peak in the cross-correlation function is caused by an eclipsing binary consisting of two late-K, or early-M stars with an orbital period of 0.435338 days. This is the eclipsing system. These two stars then orbit the K0V-primary with a period of 3.3 years. The assumption that BS Ind is a triple system consisting of a K0V star and two late-K, or early-M stars also explains the unusual brightness of the object and the near infrared excess. Thus, BS Ind is unique, as it contains by far the shortest-period young binary star, and these stars are eclipsing.

Aims.In this letter we characterise IRAS 12556–7731 as the first lithium-rich M-type giant. Based on its late spectral type and high lithium content, and because of its proximity in angular distance to the Chamaeleon II star-forming region, the star was misclassified as a young low-mass star in a previous work.

Context.HD 61005 is a nearby young solar type star that shows a large infrared excess due to a debris disk. The disk has been recently imaged from ground and space, with indications of several components. Some characteristics of the disk suggest the presence of planetary companions around the star, that remain undetected in deep adaptive optics imaging.

We present the first observations of a probable brown dwarf, obtained with the new spectrograph X-shooter mounted on the UT2@VLT. The target (2MASS J053825.4-024241) is a 0.06 M⊙object in the star-formation region σOri. The X-shooter spectrum covers simultaneously the whole range from UV to NIR (300–2500 nm). The J053825.4-024241 spectrum is rich in emission lines that are typical of accreting young object and clearly shows the Balmer jump. Moreover, many photospheric atomic and molecular absorption lines yield the spectral type and confirm that the object is young. We compute the mass accretion rate from all available observed accretion diagnostics. We find that there is a large spread in the Ṁaccvalues (up to a factor 40) that is not caused by variability; some of this spread may be intrinsic, i.e., owing to different physical conditions of the emitting region for the same Ṁacc. However, within the large error bars all Ṁaccmeasurements agree, and the mean value is logṀacc~ −9.86 ± 0.45 M⊙/y. The hydrogen Balmer lines are clearly detected up to n= 25. Their ratios suggest that the emitting region is cold (T~ 2000−3000 K), dense and in thermal equilibrium (LTE), and that the lines are optically thick up to n~ 21. We briefly discuss the implications of this result for magnetospheric accretion models.