A. Mora

EXPORT: Spectral classification and projected rotational velocities of Vega-type and pre-main sequence stars

In this paper we present the rst comprehensive results extracted from the spectroscopic campaigns car- ried out by the EXPORT (EXoPlanetary Observational Research Team) consortium. During 1998{1999, EXPORT carried out an intensive observational eort in the framework of the origin and evolution of protoplanetary sys- tems in order to obtain clues on the evolutionary path from the early stages of the pre-main sequence to stars with planets already formed. The spectral types of 70 stars, and the projected rotational velocities, v sini ,o f 45 stars, mainly Vega-type and pre-main sequence, have been determined from intermediate- and high-resolution spectroscopy, respectively. The rst part of the work is of fundamental importance in order to accurately place the stars in the HR diagram and determine the evolutionary sequences; the second part provides information on the kinematics and dynamics of the stars and the evolution of their angular momentum. The advantage of using the same observational conguration and methodology for all the stars is the homogeneity of the set of pa- rameters obtained. Results from previous work are revised, leading in some cases to completely new determinations of spectral types and projected rotational velocities; for some stars no previous studies were available.

On the simultaneous optical and near-infrared variability of pre-main sequence stars

For a complete understanding of the physical processes causing the photometric variability of pre-main sequence systems, simultaneous optical and near-IR observations are required to disentangle the emission from the stars and that from their associated circumstellar disks. Data of this sort are extremely rare and little systematic work has been reported to date. The work presented in this paper is a systematic attempt in this direction. It presents an analysis of the simultaneous optical and near-IR photometric variability of 18 Herbig Ae/Be and T Tauri stars which were observed in October 98 by the EXPORT collaboration. The time dierence between the UBVRI and JHK measurements is less than 1 hour in50% of the data and the largest dierence is around 2 hours in only10% of the data. Twelve stars appear to show a correlation between the optical and near-IR variability trends, which suggests a common physical origin such as spots and/or variable extinction. The optical and near-IR variability is uncorrelated in the rest of the objects, which suggests it originates in distinctly dierent regions. In general, the optical variability qualitatively follows the predictions of starspots or variable extinction. As far as the near-IR is concerned, the simultaneity of the observations demonstrates that for most objects the flux is largely produced by their circumstellar disks and, consequently, in many cases the near-IR fluctuations must be attributed to structural variations of such disks producing variations of their thermal emission and/or scattered light. The observed near-IR changes of up to around 1 mag on timescales of 1{2 days provide interesting challenges for understanding the mechanisms generating such remarkable variabilities, an issue insuciently investigated until now but one which deserves further theoretical and modeling eorts.

Parameters of Herbig Ae/Be and Vega-type stars ⋆

Context. This work presents the characterization of 27 young early-type stars, most of them in the age range 1‐10 Myr, and three ‐suspected‐ hot companions of post-T Tauri stars belonging to the Lindroos binary sample. Most of these objects show IR excesses in their spectral energy distributions, which are indicati ve of the presence of disks. The work is relevant in the fields o f stellar physics, physics of disks and formation of planetary systems. Aims. The aim of the work is the determination of the effective temperature, gravity, metallicity, mass, luminosi ty and age of these stars. An accurate modelling of their disks needs, as a previ ous step, the knowledge of most of these parameters, since they will determine the energy input received by the disk and hence, its geometry and global properties. Methods. Spectral energy distributions and mid-resolution spectra were used to estimate Teff, the effective temperature. The comparison of the profiles of the Balmer lines with synthetic pro files provides the value of the stellar gravity, g∗. High-resolution optical observations and synthetic spectra are used to estimate the metallicity, [M/H]. Once Teff, g∗ and [M/H] are known for each star, evolutionary tracks and isochrones provide estimations of the mass, luminosity, age and distances (or upper limits in some cases). The method is original in the sense that it is distance-indep endent, i.e. the estimation of the stellar parameters does n ot require, as it happens in other works, the knowledge of the distance to the object. Results. Stellar parameters (effective temperature, gravity, metallicity, mass, luminosi ty, age and distances ‐or upper limits) are obtained for the sample of stars mentioned above. A detailed discussion on some individual objects, in particular VV Ser, RR Tau, 49 Cet and the three suspected hot companions of post-T Tauris, is presented. Conclusions. These results, apart from their intrinsic interest, would b e extremely valuable to proceed a step further and attempt to model the disks surrounding the stars. The paper also shows the diffi culty posed by the morphology and behaviour of the system star+disk in the computation of the stellar parameters.

EXPORT: Optical photometry and polarimetry of Vega-type and pre-main sequence stars ?

This paper presents optical UBVRI broadband photo-polarimetry of the EXPORT sample obtained at the 2.5 m Nordic Optical Telescope. The database consists of multi-epoch photo-polarimetry of 68 pre-main-sequence and main-sequence stars. An investigation of the polarization variability indicates that 22 objects are variable at the 3 sigma level in our data. All these objects are pre-main sequence stars, consisting of both T Tauri and Herbig Ae/Be objects while the main sequence, Vega type and post-T Tauri type objects are not variable. The polarization properties of the variable sources are mostly indicative of the UXOR-type behaviour; the objects show highest polarization when the brightness is at minimum. We add seven new objects to the class of UXOR variables (BH Cep, VX Cas, DK Tau, HK Ori, LkH alpha 234, KK Oph and RY Ori). The main reason for their discovery is the fact that our data-set is the largest in its kind, indicating that many more young UXOR-type pre-main sequence stars remain to be discovered. The set of Vega-like systems has been investigated for the presence of intrinsic polarization. As they lack variability, this was done using indirect methods, and apart from the known case of BD+31 degrees 643, the following stars were found to be strong candidates to exhibit polarization due to the presence of circumstellar disks: 51 Oph, BD+31 degrees 643C, HD 58647 and HD 233517.

EXPORT: Near-IR observations of Vega-type and pre-main sequence stars

We present near-IR J H K photometric data of sample of 58 main-sequence, mainly Vega-type, and pre-main sequence stars. The data were taken during four observation runs in the period May 1998 to January 1999 and form part of a coordinated effort with simultaneous optical spectroscopy and photo-polarimetry. The near-IR colors of the MS stars correspond in most cases tp photospheric colors, although noticealbe reddening is present towards a few objects, and these stars show no brightness variability within the observational errors. On the other hand, the PMS stars show near-IR excesses and variability consistent with previous data.

A Herschel resolved far-infrared dust ring around HD 207129

Context. Dusty debris discs around main sequence stars are thought to be the result of continuous collisional grinding of planetesimals in the system. The majority of these systems are unresolved and analysis of the dust properties is limited by the lack of information regarding the dust location. Aims. The Herschel DUNES key program is observing 133 nearby, Sun-like stars (<20 pc, FGK spectral type) in a volume limited survey to constrain the absolute incidence of cold dust around these stars by detection of far infrared excess emission at flux levels comparable to the Edgeworth-Kuiper belt (EKB). Methods. We have observed the Sun-like star HD 207129 with Herschel PACS and SPIRE. In all three PACS bands we resolve a ring-like structure consistent with scattered light observations. Using α Bootis as a reference point spread function (PSF), we deconvolved the images, clearly resolving the inner gap in the disc at both 70 and 100 μm. Results. We have resolved the dust-producing planetesimal belt of a debris disc at 100 μm for the first time. We measure the radial profile and fractional luminosity of the disc, and compare the values to those of discs around stars of similar age and/or spectral type, placing this disc in context of other resolved discs observed by Herschel/DUNES.

Can eccentric debris disks be long-lived? A first numerical investigation & application to 2 Reticuli

Context. Imaging of debris disks has found evidence for both eccentric and o set disks. One hypothesis is that these provide evidence for massive perturbers, for example planets or binary companions, that sculpt the observed structures. One such disk was recently observed in the far-IR by the Herschel ? Space Observatory around 2 Reticuli. In contrast with previously reported systems, the disk is significantly eccentric, and the system is Gyr-old. Aims. We aim to investigate the long-term evolution of eccentric structures in debris disks caused by a perturber on an eccentric orbit around the star. We hypothesise that the observed eccentric disk around 2 Reticuli might be evidence of such a scenario. If so we are able to constrain the mass and orbit of a potential perturber, either a giant planet or binary companion. Methods. Analytical techniques are used to predict the e ects of a perturber on a debris disk. Numerical N-body simulations are used to verify these results and further investigate the observable structures that could be produced by eccentric perturbers. The long-term evolution of the disk geometry is examined, with particular application to the 2 Reticuli system. In addition, synthetic images of the disk are produced for direct comparison with Herschel observations. Results. We show that an eccentric companion can produce both the observed o sets and eccentric disks. Such e ects are not immediate and we characterise the timescale required for the disk to develop to an eccentric state (and any spirals to vanish). For the case of 2 Reticuli, we place limits on the mass and orbit of the companion required to produce the observations. Synthetic images show that the pattern observed around 2 Reticuli can be produced by an eccentric disk seen close to edge-on, and allow us to bring additional constraints on the disk parameters of our model (disk flux, extent). Conclusions. We determine that eccentric planets or stellar companions can induce long-lived eccentric structures in debris disks. Observations of such eccentric structures, thus, provide potential evidence of the presence of such a companion in a planetary system. We consider the specific example of 2 Reticuli, whose observed eccentric disk can be explained by a distant companion (at tens of AU), on an eccentric orbit (ep & 0:3).

Dynamics of the circumstellar gas in the Herbig Ae stars BF Orionis, SV Cephei, WW Vulpeculae and XY Persei

We present high resolution (λ/∆λ = 49 000) ´ spectra of the intermediate mass, pre-main sequence stars BF Ori, SV Cep, WW Wul and XY Per. The spectra cover the range 3800−5900 A and monitor the stars on time scales of months and days. All spectra show a large number of Balmer and metallic lines with variable blueshifted and redshifted absorption features superimposed to the photospheric stellar spectra. Synthetic Kurucz models are used to estimate rotational velocities, effective temperatures and gravities of the stars. The best photospheric models are subtracted from each observed spectrum to determine the variable absorption features due to the circumstellar gas; those features are characterized in terms of their velocity, v, dispersion velocity, ∆v, and residual absorption, Rmax. The absorption components detected in each spectrum can be grouped by their similar radial velocities and are interpreted as the signature of the dynamical evolution of gaseous clumps with, in most cases, solar-like chemical composition. This infalling and outflowing gas has similar properties to the circumstellar gas observed in UX Ori, emphasizing the need for detailed theoretical models, probably in the framework of the magnetospheric accretion scenario, to understand the complex environment in Herbig Ae (HAe) stars. WW Vul is unusual because, in addition to infalling and outflowing gas with properties similar to those observed in the other stars, it shows also transient absorption features in metallic lines with no obvious counterparts in the hydrogen lines. This could, in principle, suggest the presence of CS gas clouds with enhanced metallicity around WW Vul. The existence of such a metal-rich gas component, however, needs to be confirmed by further observations and a more quantitative analysis.

A dynamical study of the circumstellar gas in UX Orionis

We present the results of a high spectral resolution (== 49 000) study of the circumstellar (CS) gas around the intermediate mass, pre-main sequence star UX Ori. The results are based on a set of 10 ´ echelle spectra covering the spectral range 3800-5900 A, monitoring the star on time scales of months, days and hours. A large number of transient blueshifted and redshifted absorption features are detected in the Balmer and in many metallic lines. A multigaussian fit is applied to determine for each transient absorption the velocity,v, dispersion velocity,v, and the parameter R, which provides a measure of the absorption strength of the CS gas. The time evolution of those parameters is presented and discussed. A comparison of intensity ratios among the transient absorptions suggests a solar-like composition of the CS gas. This confirms previous results and excludes a very metal-rich environment as the cause of the transient features in UX Ori. The features can be grouped by their similar velocities into 24 groups, of which 17 are redshifted and 7 blueshifted. An analysis of the velocity of the groups allows us to identify them as signatures of the dynamical evolution of 7 clumps of gas, of which 4 represent accretion events and 3 outflow events. Most of the events decelerate at a rate of tenths of m s 2 , while 2 events accelerate at approximately the same rate; one event is seen experiencing both an acceleration and a deceleration phase and lasts for a period of few days. This time scale seems to be the typical duration of outflowing and infalling events in UX Ori. The dispersion velocity and the relative aborption strength of the features do not show drastic changes during the lifetime of the events, which suggests they are gaseous blobs preserving their geometrical and physical identity. These data are a very useful tool for constraining and validating theoretical models of the chemical and physical conditions of CS gas around young stars; in particular, we suggest that the simultaneous presence of infalling and outflowing gas should be investigated in the context of detailed magnetospheric accretion models, similar to those proposed for the lower mass T Tauri stars.

The pre-main sequence binary HK Ori - Spectro-astrometry and EXPORT data

In this paper we present multi-epoch observations of the pre-main-sequence binary HK Ori. These data have been drawn from the EXPORT data base and are complemented by high-quality spectro-astrometric data of the system. The spectroscopic data appear to be very well represented by a combination of an A dwarf star spectrum superposed on a (sub)giant G-type spectrum. The radial velocity of the system is consistent with previous determinations, and does not reveal binary motion, as expected for a wide binary. The spectral, photometric and polarimetric properties and variability of the system indicate that the active object in the system is a T Tauri star with UX Ori characteristics. The spectro-astrometry of HK Ori is sensitive down to milliarcsecond scales and confirms the speckle interferometric results from Leinert et al. The spectro-astrometry allows the identification, with fair certainty, of the active star within the binary, which we suggest to be a G-type T Tauri star based on its spectral characteristics.