Search

Survey observations were carried out in the MBM 12 (sometimes referred to as Lynds 1457) dark cloud in search of Hα emission stars by using a slitless grism spectrograph on the 2.6-m reflector of Byurakan Astrophysical Observatory, Armenia. Four new Hα stars were detected by the survey and identified as candidate T Tauri stars by follow-up low-dispersion spectroscopy and/or near-infrared photometry.

We report a detection of weak diuse interstellar bands (DIBs) in the close vicinity of 6 Herbig Ae/Be pre- main sequence stars. A common feature of these objects is the presence of a dusty shell where DIB carriers are apparently formed/destroyed. The possible influence of ultraviolet flux on carriers of di use interstellar bands is discussed as well as DIB to DIB intensity ratios in the spectra of the program stars and in the general interstellar medium.

Recently, Brittain and Rettig, using the cryogenic echelle spectrograph at the Infrared Telescope Facility to study the infrared emission from the inner preplanetary disk of the Herbig Ae/Be star HD141569, detected CO and H3+ ion emission. This emission has been tentatively interpreted as due to the existence of a forming gas giant planet. The suggested protoplanetary blob appears to be orbiting its host star at about 7 AU being perhaps 2 AU across and roughly five times the mass of Jupiter. Based on numerical modeling of the evolution of the dust disk we show that their observational results are compatible with the presence of an evolved giant vortex in the disk. Our calculations suggest that vortices formed in disks similar to the one found around HD141569 are more effective at capturing solid material than equivalent structures around solar-like stars. On the other hand, we investigate the possibility to find evidence for large-scale vortices in preplanetary disks by submillimeter interferometry. Disks around Herbig Ae/Be stars may be primary targets for giant vortex detection using this technique.

A photometric study of the G-type pre–main-sequence star CB 34V was undertaken at Van Vleck Observatory during the 2001–2002 observing season. The star remained in its bright state throughout the 5 month monitoring period, showing no evidence of the large amplitude variability that originally drew attention to it. However, it did exhibit periodic variability with an amplitude of ~0.6 mag. A highly significant peak is present in the periodogram at 1.186 days, but analysis of the light curve suggests that the actual rotation period of the star is twice that—2.372 days. Apparently, the star had similar spots on opposite hemispheres during this time and we view it close to equator-on. This circumstance allows us to place additional constraints on the radius and luminosity of the star. The new data argue against its being an FU Orionis star and in favor of its being either a UX Orionis star or an object (similar to KH 15D) that is eclipsed episodically by circumstellar matter.

We interpret the X-ray light curve obtained by XMM-Newton during a total X-ray eclipse in the nearby binary star α Coronae Borealis. This system consists of a G5 V main-sequence star orbiting an X-ray dark A0 star. The secondary G star is a young, active solar analog with an age of a few 100 Myr. As the primary A star eclipses active regions on the X-ray bright companion, the light curve drops in consecutive steps to zero; as individual active regions reappear during egress, the flux rises in similar steps. The ingress and egress light curves are combined to reconstruct the 2D distribution of X-ray brightness on and around the G star. Three different methods are applied, and variations due to statistical noise and uncertainties in the binary system parameters are discussed. Although the solutions are non-unique, all reconstructions reveal a similar distribution of X-ray bright regions and large areas with little flux. We present plausible estimates of (lower limits to) the electron densities in the bright regions, obtaining characteristic values between 10 9 -3 × 10 10 cm −3 .

We present the results of spectroscopic observations for 52 objects from the list of H α emission stars of Stephenson (1986). Out of six known T Tauri stars observed, five showed H α in emission and in one (StHa 40), Ha changed from being in absorption to emission over a period of two years, accompanied by photometric and spectral type variability. We confirm the T Tauri nature of one Stephenson object (StHa 48) on the basis of the presence of H α and H β in emission, Li I λ6708 in absorption, infrared excess and X-ray emission. Among the 52 objects observed, there were other emission line objects: 1 Ke star, 1 BQ[ ] star, 2 galaxies and 2 Be stars. We present a higher-resolution spectrum of StHa 62 showing permitted and forbidden lines in emission typical of BQ[ 1 stars. Twenty five out of 30 newly observed objects failed to show H α in emission. We also present 2MASS observations for 112 StHa objects. We suggest three Stephenson objects (StHa 52, 125 and 129) to be YSOs on the basis of 2MASS, IRAS and ROSAT observations. These and all other known YSOs amongst StHa stars are found in regions of star-forming clouds in Taurus, Orion and Ophiuchus. YSOs at high galactic latitudes in other parts of the sky are therefore rare.

We obtained high-resolution spectroscopic observations of 34 γ Doradus candidates. From the red-wavelength spectra, we determined spectral classes, radial velocities, and projected rotational velocities. The spectra of seven late A or early F stars show metallic lines that have composite profiles consisting of a narrow component near the center of a broad line, indicating that they may be shell stars or binaries. Several stars, including HD 152896, HD 173977, HD 175337, and HD 195068/9, show large line profile asymmetries. Two stars, HD 11443 (=α Trianguli) and HD 149420, are ellipsoidal variables and not γ Doradus stars. The percentage of binary systems in our sample may be as high as 74%.

We present the results of our simultaneous photometric and polarimetric observations of the Herbig Ae/Be star VX Cas acquired in 1987–2001. The star belongs to the UX Ori subtype of young variable stars and exhibits a rather low level of photometric activity: only six Algol-like minima with amplitudes ΔV>1m were recorded in 15 years of observations. Two of these minima, in 1998 and 2001, were the deepest in the history of the star’s photometric studies, with V amplitudes of about 2m. In each case, the dimming was accompanied by an increase in the linear polarization in agreement with the law expected for variable circumstellar extinction. The highest V polarization was about 5%. Observations of VX Cas in the deep minima revealed a turnover of the color tracks, typical of stars of this type and due to an increased contribution from radiation scattered in the circumstellar disk. We separated the observed polarization of VX Cas into interstellar (P is) and intrinsic (P in) components. Their position angles differ by approximately 60°, with P is dominating in the bright state and P in dominating during the deep minima. The competition of these two polarization components leads to changes in both the degree and position angle of the polarization during the star’s brightness variations. Generally speaking, in terms of the behavior of the brightness, color indices, and linear polarization, VX Cas is similar to other UX Ori stars studied by us earlier. A number of episodes of photometric and polarimetric activity suggest that, in their motion along highly eccentric orbits, circumstellar gas and dust clouds can enter the close vicinity of the star (and be disrupted there).

We present TIMMI2 observations at 10 μ m of 12 fields centered on known Herbig AeBe (HAeBe) stars. We detected sources (in addition to the HAeBe star) in five fields, three of which are associated with early B stars. Using complementary near-IR and optical data, we estimate that the detected sources are young, embedded ( A V $\sim 5{-}24$ mag), intermediate-mass objects (~$1.5{-}3.5$ $M_\odot$). In four objects the 10 μ m emission can be accounted for by a circumstellar disk, while in 2 cases (possibly 3) there is evidence that the objects are still in an earlier stage of evolution (Class I). These results confirm the association of the early B Herbig stars with rather rich clusters, as well as their very young age. If our estimates of the stellar parameters are confirmed, this would be the largest sample so far of embedded, intermediate-mass objects available for further studies.

New spectroscopic observations obtained in 1998–2001 and published radial velocities of neutral helium absorption lines suggest that the Herbig Ae/Be star HD 200775 is a spectroscopic binary with a 1180±60 day period. We have determined the elements of the spectroscopic orbit. The presence of both rapid and long-time-scale spectral variability is demonstrated. The rapid variability is apparently due to a nonuniform circumstellar environment and modulation by the axial rotation of the system’s primary. Using data from the literature, we plot the V light curve of the system, which shows some brightening near the time of the minimum radial velocity, with its duration not exceeding 0.2 of the orbital cycle. We suggest that the system’s secondary is a low-mass (≥ 0.5M⊙) protostar, and that a semitransparent extended circumstellar envelope gives rise to eclipses in the system.

We present new mid-infrared spectroscopy of the emission from warm circumstellar dust grains in Herbig Ae/Be stars. Our survey significantly extends the sample that was studied by Bouwman et al. (2001). We find a correla- tion between the strength of the silicate feature and its shape. We interpret this as evidence for the removal of small (0.1m) grains from the disk surface while large (1-2m) grains persist. If the evolution of the grain size distribution is dominated by gravitational settling, large grains are expected to disappear first, on a timescale which is much shorter than the typical age of our programme stars. Our observations thus suggest a continuous replenishment of micron sized grains at the disk surface. If the grain replenishment is due to the dredge-up of dust from the disk interior, the mineralogy we observe is representative of the bulk composition of dust in these stars.

We report the discovery of periodicities in the light curve of the Pre-Main Sequence (PMS) Herbig Ae star V346 Ori. We interpret variations in the light curve of the star as the superposition of at least two periodic signals that result from δ Scuti-type stellar oscillations. The computation of linear non-adiabatic radial pulsation models for PMS stars reproduces these periods for a 1.55 M O . star with T e f f = 7410 K and log L/L O . = 0.74 pulsating in the fundamental and in the second overtone. If our assumption of radial oscillations is indeed correct, the star's location on the HR diagram places it at a distance significantly smaller than that of the Orion star forming region, hence raising questions regarding the evolutionary status of V346 Ori. Observations with better time sampling and spanning a longer period of time are needed if the latter question is to be addressed fully.

I summarize the method commonly used to estimate the current star formation rate per unit volume as a function of redshift from various star formation diagnostics, as well as the often conflicting results from the various multi-wavelength datasets collected in recent years. Combined together, and with the help of theoretical and empirical models, these results allow us to address the issues of excitation, reddening, metallicity, and star formation history in individual galaxies at low redshift. They seem to point to the fact, that a lot of the star formation in the local universe, and potentially at all redshift, occurs in burst mode rather than continuously.

The amount of excess emission in the near-infrared spectrum of classical T Tauri stars (CTTS) is higher than predicted by current models. This lack of understanding about the CTTS systems impinges directly on our ability to achieve a deeper knowledge of the final stages of low mass stars formation, of the pre-main sequence history of the Sun and even on the processes that lead to planet formation. Here I present a portion of the Unsolved Universe of the CTTS systems and some Challenges for the Future in this field.

Spectroscopic and photometric observations of objects embedded in the L1340 dark cloud are reported. An entire series of HH-objects in the RN07 region was discovered, including new ones that form two or three flows. It is suspected that several cases of HH emission are included. In addition, 14 emission stars, of which 11 are new, were found in this region. Most of these stars are localized inside the RN07 nebula which, therefore, encompasses a compact cluster of very young stars. Other nebulous objects inside the L1340 cloud, many of which are associated with IR sources, are described. As a whole, this cloud represents an extremely active star forming region.

We present spectroscopic observations of 33 emission line stars from the Sanduleak & Stephenson's (1973) list. This work is part of a program to investigate emission-line objects in the southern hemisphere whose nature is not well established in the literature. The objects were observed at two different spectral regions providing full coverage in the 3100–8700 A interval. In this paper we describe the main spectroscopic features and discuss the nature of the objects. It is proposed that 16 of them are Be stars, 5 are peculiar Be stars, 4 are T Tauri stars, 7 are M-type stars with emission-lines and 1 is a Herbig object.

Spectral observations of the Herbig Ae star MWC 480 are reported. Observations were made on the 2.6 m telescope at the Crimean Astrophysical Observatory and the 6 m telescope at the Special Astrophysical Observatory in the neighborhoods of the sodium resonance doublet, the He I λ5876 A line, the oxygen O I λ7774 A line, the Hα line, and some others. The Hα line has a P Cyg-type profile which is typical of anisotropic decelerated material outflows. The parameters of the line profile vary on a time scale on the order of days or longer. The blue wing of the line profile, in which noticeable changes are detectable over times of a few hours, is subject to the greatest variation. An unusual line shape is observed in the sodium lines. Their profiles resemble type P Cyg profiles with discrete absorption components can be seen in the blue wing. The number, shape, and radial velocities of the components change with time. The maximum radial velocity is -330 km/s and the minimum, about -50 km/s. The high velocity components are subject to the greatest variability. An analysis of the spectral variability yields the following conclusions: (1) the inner layers of the accretion disk of MWC 480 reach right to the star. The maximum rotation velocity of the circumstellar gas derived from the oxygen OI 7774 A line shape is close to 400-500 km/s, which corresponds roughly to the radius of the last Keplerian orbit. (2) A highly nonuniform, high velocity component of the disk wind, which contains dense fragments (microjets), develops in this region. They appear to form as a result of the unstable structure of the magnetic field in the layers of the accretion disk closest to the star. (3) The maximum velocities of the microjets are only slightly higher than the escape velocity at the star's surface. Thus, the bulk of the momentum which they acquire is expended in overcoming the star's gravity and this causes a deceleration in the radial motion of the gas. This kind of structure for the radiating region is consistent with magneto-centrifugal models of the disk wind in which the intrinsic magnetic field of the accretion disk plays a dominant role in the acceleration of the gas.

This paper begins a new series of studies devoted to a critical re-examination of the role of duplicity for the Be phenomenon and for the variability patterns observed for many Be stars. Based on both dynamical and energy considerations and a numerical gas-dynamical modelling, a new hypothesis of the forma- tion of Be envelopes in binaries, via an outflow from a rapidly rotating B star in a detached binary, is outlined. It is shown that such an outflow is facilitated by the presence of a companion to the B star and leads to the formation of an envelope but not to any significant mass exchange between the binary components.

In order to increase the completeness of the investigations of stellar abundances, we can use spectrum synthesis method, new atomic data and observation of stellar spectra with resolution comparable to solar spectral atlases. We made a brief review of main problems of these three ways. We present new results of abundance determinations in the atmospheres of four stars. The first is the implementation of new atomic data to well known Przybylski`s star. We show that the number of spectral lines, which can be identificated in the spectrum of this star, can be significantly higher. The second example is the investigation of Cyg. We found the abundances of 51 elements in the atmosphere of this mild barium star. The third example is halo star HD221170. Our preliminary abundance pattern consists of 42 elements. The heaviest elements in this pattern are U and Th. The last star is the spectroscopic binary HD153720. The number of elements investigated in the spectra of components of this star is not large, but the results show that the components are Am-stars.

We present the infrared spectra of Herbig Ae/Be stars including the Infrared Space Ob-servatory (ISO) data. To investigate the overall properties of their circumstellar dust envelope and/or disk, we combine the IR spectra with photometric data ranging from the UV through the optical into the sub-mm region. We study the general characteristics of the spectral energy distributions using simple analysis. We plot the positions on the HR-diagram to compare with the theoretical pre-main-sequence evolution tracks.

We searched the EROS1 database consisting of 80 000 stars in the bar of the Large Magellanic Cloud for blue objects with irregular photometric behavior similar to Galactic Herbig Ae/Be (HAeBe) stars. We found 14 new objects in addition to the discovery of 7 HAeBe candidate stars found in our previous studies. Based on their light curve and colours we classify these new objects also as EROS LMC HAeBe Candidates (ELHCs). The objects are located in an area of enhanced 60m emission, which indicates a star forming region. The stars are concentrated in three areas and the majority of the objects are clustered round the N 120 nebular complex. The light curves are interpreted with a simple model, involving obscuration by an inhomogeneous orbiting circumstellar dust cloud as the dominant cause of the variability. We find that the dust orbiting the stars is at a radial distance in the order of 10 to a few 100 AUs. Most of the objects are "bluer-when-fainter". This is attributed to the presence of a (blue) scattering nebula with a size smaller than the point spread function of the photometric images. The stellar parameters are derived from the photometry. The luminosity of the stars is in the range of 3:4< log(L=L)< 4: 2a nd the colours indicate spectral types O or B. The stars are located in the HR-diagram above the birthline for Galactic stars by about 1 dex in luminosity. The method used to detect the ELHCs stars on optical variability criteria is discussed. We show that our sample of HAeBe candidates in the EROS1 field of view is complete up to V' 16:5 m for the typical amplitude and time scale of the ELHCs.

A preliminary search for stars that may have formed coevally with the apparently young halo B-type star PHL 346 has been performed with the 2dF multifibre spectrograph on the Anglo-Australian Telescope (AAT). Candidates were selected for spectroscopy from APM scans of B and R Schmidt plates centred on PHL 346. A total of 476 stars of spectral type A or F were found; radial velocity estimates and more accurate spectral type assignments narrowed the number of possible coeval candidates to 6 A-type and 14 F-type stars. A statistical analysis of these results using a comparison with a control field suggests that the number of A-type or F-type candidate stars around PHL 346 is not unexpected, and that they need not be associated with PHL 346. A number of ways to improve the project are suggested.

We present our photometric and spectroscopic observations of the hot emission-line high-latitude (b=+64°) star discovered by Haro. The star exhibited no light variability during 1993–2001. Based on our observations and on the detailed study of the star by Herbig (1992), we improved its parameters, T=22000 K and logg=4.2, and inferred its evolutionary status. We show that Haro’s star does not belong to the class of protoplanetary objects but is most likely a hot subdwarf of the Galactic halo with an emission-line spectrum formed in the outer layers of the star or in its stellar wind.

We report the results of an XMM-Newton observation of the Lagoon Nebula (M 8). Our EPIC images of this region reveal a cluster of point sources, most of which have optical counterparts inside the very young open cluster NGC 6530. The bulk of these X-ray sources are probably associated with low and intermediate mass pre-main sequence stars. One of the sources experienced a flare-like increase of its X-ray flux making it the second brightest source in M 8 after the O4 star 9 Sgr. The X-ray spectra of most of the brightest sources can be fitted with thermal plasma models with temperatures of kT af ew keV. Only a few of the X-ray selected PMS candidates are known to display H emission and were previously classified as classical T Tauri stars. This suggests that most of the X-ray emitting PMS stars in NGC 6530 are weak-line T Tauri stars. In addition to 9 Sgr, our EPIC field of view contains also a few early-type stars. The X-ray emission from HD 164816 is found to be typical for an O9.5 III-IV star. At least one of the known Herbig Be stars in NGC 6530 (LkH 115) exhibits a relatively strong X-ray emission, while most of the main sequence stars of spectral type B1 and later are not detected. We also detect (probably) diuse X-ray emission from the Hourglass Region that might reveal a hot bubble blown by the stellar wind of Herschel 36, the ionizing star of the Hourglass Region.

We have obtained high spatial resolution mid-IR observations of four intermediate-mass pre–main-sequence stars: HD 259431, AS 310, LkHα 234, and MWC 1080. Our observations reveal the morphology of the circumstellar (500–10,000 AU) environments of these young stars. We resolve companion stars and complex diffuse emission from dust that can account for on average 50% of the IR emission detected in previous low-resolution observations. We calculate new spectral indices for these objects and show that they differ from those expected from simple geometrically flat disk models. We also show mid-IR spectral energy distributions that reveal the presence of silicate emission and absorption features for several objects. Finally, temperature maps constructed from our observations suggest the presence of transiently heated grains, and optical depth maps point to the locations of the most embedded objects.

I present a literature survey of the currently available Doppler images of cool stars. The 65 individual stars with Doppler images consist of 29 single stars and 36 components in close binaries. Out of the total, 31 were observed only once but 12 stars are (or were) being monitored for years. Each image for each star is identified with the time when it was observed, whether photometry was used in the imaging, the inclination of the stellar rotation axis, the v sin i, the stellar rotation period, and whether a polar spot and/or a high-latitude or low-latitude spot was seen. The type of variable star and its M-K spectral classification is also listed to identify the evolutionary status. The sample consists of 3 classical T Tauri stars, 8 weak-lined T Tauri's, 27 main-sequence stars, 9 subgiants, and 18 giants. The total number of Doppler images is 245 as of June 2002.

We present 318 high-dispersion echelle spectra and 450 UBVRCIC observations of the pre-main-sequence star V351 Orionis. These data were obtained simultaneously over a two-week period. The star is unusual in that it no longer shows the frequent sharp fadings that occurred prior to 1986. We find features characteristic of Herbig Ae stars: Hα emission, sharp and broad Na ii absorption components and weak He i absorption. The Hα profile is strongly variable with a time-scale of 1 d and normally is an inverse P Cygni profile, but on occasions overt emission disappears altogether. The equivalent widths of the sharp Na ii absorption lines have decreased quite markedly since 1995, indicating a decrease in the column density of gas in the line of sight. We also detect very weak forbidden [O i] emission. There is no evidence for veiling of the star, but a possible 1.0-d or 0.5-d period in the equivalent widths of the lines could indicate the presence of an accretion hotspot. The star is of particular interest because it is pulsating. We detect at least two periods in the light variation and a further period from the radial velocity variations. Attempts at photometric mode identification were not successful, but from the morphology of the line profiles we can identify one mode as non-radial. In addition, we see on some nights several residual absorption subfeatures moving across the line profile from blue to red. These are almost certainly caused by one or more modes of high degree (l≈8).

We present the results of optical and near-IR spectroscopic and broadband multicolour photometric observations of the emission-line object V669 Cep. We nd evidence that it contains a hot, low luminosity, B4{ B6 star and a cool companion (most likely late-type giant). Signicant variations of the H line strength are detected on a timescale of months. The emission-line spectrum and strong IR-excess indicate a large amount of circumstellar gas and dust in the system. The spectral energy distribution in the near-IR region and the absence of late-type star features in the optical spectrum indicates that the cool star is heavily obscured by circumstellar dust, while the hot star is much less aected by reddening. The system is located at 1{1.5 kpc from the Sun in the local spiral arm. We suggest that V669 Cep is an evolved and probably mass exchanging binary system, a member of the group of Be stars with warm dust.

Contemporaneous spectroscopic and photometric BV observations of the RS CVn type eclipsing binary RT Lacertae were performed in summer 2000. The photometric observations were obtained at the Ege University Observatory, while the spectroscopic ones were carried out at Catania Astrophysical Observatory in the spectral range 5860-6700 Å. We obtained a high quality radial velocity curve of the system that allowed us to give more accurate values of the orbital parameters. A steady decrease of the barycentric velocity from 1920 to 2000 has been pointed out and has been discussed in the context of a third body hypothesis. Through the subtraction of a "synthetic" spectrum, built up with spectra of inactive standard stars, we detected H? excess emission which fills in the photospheric absorption profiles of both components. With the exception of a few spectra, taken close to the eclipses, in which some extra absorption or a faint double-peaked broad emission appears, there is no further evidence of circumstellar matter in this system, as suggested in previous works. The hotter and more massive star appears also as the more active at a chromospheric level, since it has a H? flux about ten times greater than the companion, on average. Rotational modulation of the H? emission has been detected in both stars. The hemisphere of the more massive star facing the observer at phase 0p.75 appears brighter (in H?) than that seen at phase 0p.25, while for the less massive G9 IV star the maximum H? emission is seen around phase 0p.0-0p.1. the analysis of the contemporaneous light curve (Lanza et al. 2002), the more massive G5 IV star results to be more active than the companion at a photospheric level, in agreement with the chromospheric behaviour observed in H?. In addition, the starspots of the G5 IV star are mainly located in the H? brighter hemisphere, suggesting a close spatial association of spots and plages in this star. The G9 IV star displays instead the maximum H? emission at the phase of maximum visibility of the smaller spotted area found from the light-curve analysis. The minimum H? emission occurs when the more heavily spotted region is visible.

We present a study of the L1340 molecular cloud, which is known to be actively forming low- and intermediate-mass stars in three independent cores. We present optical and near-infrared images of the central regions of the three cores of this cloud, better known as RNO 7, RNO 8, and RNO 9. We show that RNO 7 is a Herbig Be cluster. We have discovered three Herbig-Haro flows in core A (southwestern part) of the cloud, named HH 487, 488, and 489. HH 487 is a spectacular set of bow shocks that appear to be driven by IRAS 02224+7227. HH 488A is a ~34-long flow possibly overlapping a second flow (HH 488B) in the line of sight. HH 489 is a flow extending to ~1' on either side of the driving source IRAS 02249+7230. Most of these Herbig-Haro objects are found to be high-excitation objects.

Photometric observations in the near infrared for 161 S stars, including 18 Tc-rich (intrinsic) stars, 19 Tc-deficient (extrinsic) ones and 124 candidates for Tc-deficient S stars, are presented in this paper. Based on some further investigations into the infrared properties of both Tc-rich and Tc-deficient S stars, 104 candidates are identified as very likely Tc-deficient S stars. The large number of infrared-selected Tc-deficient S stars provides a convenient way to study the physical properties and the evolutionary status of this species of S stars.

We find that five sources listed in the new carbon star catalog are not really carbon-rich objects but oxygen-rich stars, because they all have the prominent 10mum silicate features in absorption and the 1612 MHz OH maser emission or/and the SiO molecular features. These objects were considered as carbon stars in the catalog based only on their locations in the infrared two-color diagram. Therefore to use the infrared two-color diagram to distinguish carbon-rich stars from oxygen-rich stars must be done with caution, because, in general, it has only a statistical meaning.

In this study, we compute theoretical line proles of a non-radially pulsating star, taking the non- adiabatic eects into account. These non-adiabatic eects are especially important in the atmosphere, where the spectral lines are formed, and must be accounted for. In this rst paper of the series, we present a new treatment of the perturbed thermal and dynamical equations in the atmosphere of a pulsating star. We apply our formalism to the computation of non-adiabatic eigenfunctions in a typical Cephei star with low order p-modes and in a typical slowly pulsating B star with high-order g-modes.

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.

The rotation of T Tauri stars is thought to be slowed by coupling between the stellar magnetosphere and the circumstellar disk. Stassun and collaborators have recently questioned this picture, based on their observations of young stars in and near the Orion Nebula Cluster (ONC) that indicate little evidence for disk-mediated spin-down. However, stellar spin-down can occur no faster than the rate at which angular momentum is removed from the inner disk, either through viscous processes associated with accretion or by a wind. Observational estimates of these processes suggest that the timescales for angular momentum loss may be comparable to the ages of the ONC stars, so that disk braking may not be completely effective, at least among the lowest mass stars. Testing this hypothesis will require further measurements of mass accretion and mass-loss rates as well as determinations of rotational periods as a function of age.

We make a statistical study of the energy sources of high-velocity phenomena, Herbig-Haro (HH) objects. IRAS counterparts of HH objects are identified. Their colors, brightness, geometric relation to the HH objects and SED are analysed. The sources are found to be concentrated in a band-shaped region in the IRAS color-color diagram. We suggest an explanation of thick surrounding material for this distribution. We propose a new method for identifying the energy sources based on color and brightness. This method is applied to more than 200 HH objects whose energy sources are still unknown. Finally, a group of very young stellar object candidates which have similar properties to the HH energy sources is picked out. Their large-scale distribution is discussed.

Recently, the line profile variability (lpv) of two low-v sin i Be stars, μ Cen and ω (28) CMa was successfully modelled as nonradial pulsation (nrp) of rapidly rotating stars seen pole-on. In this work, it is shown that the lpv of low-v sin i early-type Be stars in general closely resembles these two cases, and is therefore explainable by the same mechanism. The lpv of intermediate to high-v sin i Be stars can be explained by the same model if the inclination angle of the model alone is increased. Consequently, early-type Be stars form a distinct, fairly homogeneous class of non-radial low-order g-mode pulsators.

The results of photoelectric observations of the Herbig Ae/Be star MWC 1080 in the Stromgren system are given. It was found that MWC 1080 undergoes rapid rises and falls in brightness with an amplitude of up to 0m.25, occurring in a time interval of from several minutes to several hours. It is shown that MWC 1080 is similar to MWC 342 and MWC 419. It is suggested that there is a fundamental difference between the mechanisms of brightness variation of these three objects and of Herbig Ae/Be stars.

A grid of isochrones, covering a wide range of stellar ages from the zero-age main sequence to 10 billion years, is calculated in the present work on the basis of the model stars of Claret and Gimenez with allowance for convective overshoot and mass loss by the components. The ages of 88 eclipsing variables on the main sequence from Andersen's catalog and 100 chromospherically active stars from Strassmeier's catalog are calculated with a description of the method of optimum interpolation. Comparisons with age determinations by other authors are given and good agreement is established.

In the course of photopolarimetric monitoring of the Herbig Ae star WW Vul, an unusual variation of linear polarization upon energence from a deep minimum was recorded, which cannot be explained by the standard mechanism for such objects (UX Ori stars): a variable contribution of radiation scattered by circumstellar dust. An analysis of possible causes of the anomalous behavior of polarization shows that it was most likely due to the presence of aligned, aspherical dust grains in the dust cloud crossing the line of sight at that time.

The results of an investigation of XY Per based on photoelectric observations in the Stromgren system are presented. It is shown that the brighter component in the binary system XY Per AB, classified earlier as A2 II, is a variable of the Herbig Ae/Be type. At the same time, our estimates gave M V (XY Per A) = +1.25. An analysis of the variation of the indices β and c showed that XY Per A is a typical shell star: along with intense hydrogen absorption lines it displays an appreciable emission deficit in the Balmer continuum. The most noteworthy are the results of observations indicating that as the brightness decreases, the opacity of the shell increases both in lines and in the Balmer continuum. This observational fact can scarcely be explained in terms of the model of brightness variation due to dust clouds revolving around the star.

Context. In multiple pre-main-sequence systems the lifetime of circumstellar discs appears to be shorter than around single stars, and the actual dissipation process may depend on the binary parameters of the systems. Aims. We report high spatial resolution observations of multiple T Tauri systems at optical and infrared wavelengths. We determine whether the components are gravitationally bound and orbital motion is visible, derive orbital parameters, and investigate possible correlations between the binary parameters and disc states. Methods. We selected 18 T Tau multiple systems (16 binary and two triple systems, yielding 16 + 2 × 2 = 20 binary pairs) in the Taurus-Auriga star-forming region from a previous survey, with spectral types from K1 to M5 and separations from 0.22″ (31 AU) to 5.8″ (814 AU). We analysed data acquired in 2006–07 at Calar Alto using the AstraLux lucky imaging system, along with data from SPHERE and NACO at the VLT, and from the literature. Results. We found ten pairs to orbit each other, five pairs that may show orbital motion, and five likely common proper motion pairs. We found no obvious correlation between the stellar parameters and binary configuration. The 10 μ m infra-red excess varies between 0.1 and 7.2 mag (similar to the distribution in single stars, where it is between 1.7 and 9.1), implying that the presence of the binary star does not greatly influence the emission from the inner disc. Conclusions. We have detected orbital motion in young T Tauri systems over a timescale of ≈ 20 yr. Further observations with even longer temporal baseline will provide crucial information on the dynamics of these young stellar systems.

We assess the evolutionary status of EXors. These low-mass, pre-main-sequence stars repeatedly undergo sharp luminosity increases, each a year or so in duration. We place into the HR diagram all EXors that have documented quiescent luminosities and effective temperatures, and thus determine their masses and ages. Two alternate sets of pre-main-sequence tracks are used, and yield similar results. Roughly half of EXors are embedded objects, i.e., they appear observationally as Class I or flat-spectrum infrared sources. We find that these are relatively young and are located close to the stellar birthline in the HR diagram. Optically visible EXors, on the other hand, are situated well below the birthline. They have ages of several Myr, typical of classical T Tauri stars. Judging from the limited data at hand, we find no evidence that binarity companions trigger EXor eruptions; this issue merits further investigation. We draw several general conclusions. First, repetitive luminosity outbursts do not occur in all pre-main-sequence stars, and are not in themselves a sign of extreme youth. They persist, along with other signs of activity, in a relatively small subset of these objects. Second, the very existence of embedded EXors demonstrates that at least some Class I infrared sources are not true protostars, but very young pre-main-sequence objects still enshrouded in dusty gas. Finally, we believe that the embedded pre-main-sequence phase is of observational and theoretical significance, and should be included in a more complete account of early stellar evolution.

We present multiple epochs of near-IR spectroscopy for a sample of 25 young stars, including T Tauri, Herbig Ae/Be, and FU Ori objects. Using the FSPEC instrument on the Bok 90-inch telescope, we obtained K-band spectra of the BrGamma transition of hydrogen, with a resolution of ~3500. Epochs were taken over a span of >1 year, sampling time-spacings of roughly one day, one month, and one year. The majority of our targets show BrGamma emission, and in some cases these are the first published detections. Time-variability is seen in approximately half of the targets showing BrGamma emission. We compare the observed variability with expectations for rotationally-modulated accretion onto the central stars and time-variable continuum emission or extinction from matter in the inner disk. Our observations are not entirely consistent with models of rotationally-modulated magnetospheric accretion. Further monitoring, over a larger number of epochs, will facilitate more quantitative constraints on variability timescales and amplitudes, and a more conclusive comparison with theoretical models., Comment: 16 pages in emulateapj format. Accepted for publication in MNRAS

Herbig Ae/Be stars lie in the mass range between low and high mass young stars, and therefore offer a unique opportunity to observe any changes in the formation processes that may occur across this boundary. This paper presents medium resolution VLT/X-Shooter spectra of six Herbig Ae/Be stars, drawn from a sample of 91 targets, and high resolution VLT/CRIRES spectra of five Herbig Ae/Be stars, chosen based on the presence of CO first overtone bandhead emission in their spectra. The X-Shooter survey reveals a low detection rate of CO first overtone emission (7 per cent), consisting of objects mainly of spectral type B. A positive correlation is found between the strength of the CO v=2-0 and Br {\gamma} emission lines, despite their intrinsic linewidths suggesting a separate kinematic origin. The high resolution CRIRES spectra are modelled, and are well fitted under the assumption that the emission originates from small scale Keplerian discs, interior to the dust sublimation radius, but outside the co-rotation radius of the central stars. In addition, our findings are in very good agreement for the one object where spatially resolved near-infrared interferometric studies have also been performed. These results suggest that the Herbig Ae/Be stars in question are in the process of gaining mass via disc accretion, and that modelling of high spectral resolution spectra is able to provide a reliable probe into the process of stellar accretion in young stars of intermediate to high masses., Comment: Accepted for publication in MNRAS. 14 pages, 5 figures

'Abd al-Raḥmān al-Ṣūfī (AD 903–986) is justly famous for his Book of the Fixed Stars. This is an outstanding Medieval treatise on astronomy that was written in AD 964. This work was developed from Ptolemy’s Almagest, but was based upon al-Ṣūfī’s own stellar observations. The Book of the Fixed Stars has been copied down through the ages, and currently 35 copies are known to exist in various archival repositories around the world.

We present a mini-survey of Galactic B[e] stars mainly undertaken with the Large Binocular Telescope (LBT). B[e] stars show morphological features with hydrogen emission lines and an infrared excess, attributed to warm circumstellar dust. In general, these features are assumed to arise from dense, non-spherical, disc-forming circumstellar material in which molecules and dust can condensate. Due to the lack of reliable luminosities, the class of Galactic B[e] stars contains stars at very different stellar evolutionary phases like Herbig AeBe, supergiants or planetary nebulae. We took near-infrared long-slit K-band spectra for a sample of Galactic B[e] stars with the LBT-LUCI 1. Prominent spectral features, such as the Brackett γ line and CO band heads are identified in the spectra. The analysis shows that the stars can be characterized as evolved objects. Among others we find one luminous blue variable candidate (MWC314), one supergiant B[e]candidate with 13CO (MWC 137), and in two cases (MWC 623 and AS 381) indications for the existence of a late-type binary companion, complementary to previous studies. For MWC 84, IR spectra were taken at different epochs with LBT-LUCI 1 and the GNIRS spectrograph at the Gemini North telescope. The new data show the disappearance of the circumstellar CO emission around this star, previously detectable over decades. Also no signs of a recent prominent eruption leading to the formation of new CO disc emission are found during 2010 and 2013., Facultad de Ciencias Astronómicas y Geofísicas, Instituto de Astrofísica de La Plata

We used the Ultraviolet and Visual Echelle Spectrograph of the ESO-Very Large Telescope to obtain a high resolution and high signal-to-noise ratio spectrum of Stock 16-12, an early-type star which previous Delta-a photometric observations suggest being a chemically peculiar (CP) star. We used spectral synthesis to perform a detailed abundance analysis obtaining an effective temperature of 8400 +/- 400 K, a surface gravity of 4.1 +/- 0.4, a microturbulence velocity of 3.4 +0.7/-0.3 km/s, and a projected rotational velocity of 68 +/- 4 km/s. We provide photometric and spectroscopic evidence showing the star is most likely a member of the young Stock 16 open cluster (age 3-8 Myr). The probable cluster membership, the star's position in the Hertzsprung-Russell diagram, and the found infrared excess strongly suggest the star is still in the pre-main-sequence (PMS) phase. We used PMS evolutionary tracks to determine the stellar mass, which ranges between 1.95 and 2.3 Msun, depending upon the adopted spectroscopic or photometric data results. Similarly, we obtained a stellar age ranging between 4 and 6 Myr, in agreement with that of the cluster. Because the star's chemical abundance pattern resembles well that known of main sequence CP metallic line (Am) stars, the object sets important constraints to the diffusion theory. Additional spectroscopic and spectropolarimetric data allowed us to conclude that the object is probably a single non-magnetic star., Accepted for publication in MNRAS; 8 pages, 5 figures, 1 table

Variability in emission lines is a characteristic feature in young stars and can be used as a tool to study the physics of the accretion process. Here we present a study of H{\alpha} variability in 15 T Tauri and Herbig Ae stars (K7-B2) over a wide range of time windows, from minutes, to hours, to days, and years. We assess the variability using linewidth measurements and the time series of line profiles. All objects show gradual, slow profile changes on time-scales of days. In addition, in three cases there is evidence for rapid variations in H{\alpha} with typical time-scales of 10 min, which occurs in 10% of the total covered observing time. The mean accretion-rate changes, inferred from the line fluxes,are 0.01-0.07 dex for time-scales of < 1 hour, 0.04-0.4 dex for time-scales of days, and 0.13-0.52 dex for time-scales of years. In Costigan et al. 2012 we derived an upper limit finding that the intermediate (days) variability dominated over longer (years) variability. Here our new results, based on much higher cadence observations, also provide a lower limit to accretion-rate variability on similar time-scales (days), thereby constraining the accretion rate variability physics in a much more definitive way. A plausible explanation for the gradual variations over days is an asymmetric accretion flow resulting in a rotational modulation of the accretion-related emission, although other interpretations are possible as well. In conjunction with our previous work, we find that the time-scales and the extent of the variability is similar for objects ranging in mass from ~ 0.1 to ~ 5 Msol. This confirms that a single mode of accretion is at work from T Tauri to Herbig Ae stars - across a wide range of stellar masses., Comment: 16 pages (appendix a further 18 pages), 25 figures, Accepted MNRAS; References and contact email updated