The circumstellar disc around the Herbig AeBe star HD169142

We present 7 mm and 3.5 cm wavelength continuum observations towards the Herbig AeBe star HD169142 performed with the Very Large Array (VLA) with an angular resolution of ≃1 arcsec. We find that this object exhibits strong (≃4.4 mJy), unresolved (≲1 arcsec) 7 mm continuum emission, being one of the brightest isolated Herbig AeBe stars ever detected with the VLA at this wavelength. No emission is detected at 3.5 cm continuum, with a 3σ upper limit of ≃0.08 mJy. From these values, we obtain a spectral index α≳ 2.5 in the 3.5 cm to 7 mm wavelength range, indicating that the observed flux density at 7 mm is most likely dominated by thermal dust emission coming from a circumstellar disc. We use available photometric data from the literature to model the spectral energy distribution (SED) of this object from radio to near-ultraviolet frequencies. The observed SED can be understood in terms of an irradiated accretion disc with low mass accretion rate,     , surrounding a star with an age of ≃10 Myr. We infer that the mass of the disc is ≃0.04 M_⊙, and is populated by dust grains that have grown to a maximum size of 1 mm everywhere, consistent with the lack of silicate 10 μm emission. These features, as well as indications of settling in the wall at the dust destruction radius, led us to speculate that the disc of HD169142 is in an advanced stage of dust evolution, particularly in its inner regions.     

Herbig Ae/Be stars

Herbig Ae/Be stars are the higher mass counterparts of the T Tauri stars. In comparison with the latter, however, relatively little is known about them. After a historical introduction, we briefly review their optical and UV spectroscopic properties. We consider the evidence for and against disks around Herbig Ae/Be stars; the existence of which remains highly controversial. We also examine in-depth their interaction with the surrounding medium as manifested through optical outflows. It is shown that although there are similarities with analogous outflows from lower mass young stars, those from Herbig Ae/Be stars may be more poorly collimated. Jets, however, are found in at least some cases.     

HARPS spectropolarimetry of Herbig Ae/Be stars

Our knowledge of the presence and the strength of magnetic fields in intermediate‐mass pre‐main‐sequence stars remains very poor. We present new magnetic field measurements in six Herbig Ae/Be stars observed with HARPS in spectropolarimetric mode. We downloaded from the European Southern Observatory (ESO) archive the publically available HARPS spectra for six Herbig Ae/Be stars. Wavelength shifts between right‐ and left‐hand side circularly polarised spectra were interpreted in terms of a longitudinal magnetic field 〈B_z〉, using the moment technique introduced by Mathys. The application of the moment technique to the HARPS spectra allowed us in addition to study the presence of the crossover effect and quadratic magnetic fields. Our search for longitudinal magnetic fields resulted in first detections of weak magnetic fields in the Herbig Ae/Be stars HD 58647 and HD 98922. Further, we confirm the previous tentative detection of a weak magnetic field in HD 104237 by Donati et al. and confirm the previous detection of a magnetic field in the Herbig Ae star HD 190073. Surprisingly, the measured longitudinal magnetic field of HD 190073, 〈B_z〉 = 91 ± 18 G at a significance level of 5σ is not in agreement with the measurement results of Alecian et al. (2013), 〈B_z〉 = –10 ± 20 G, who applied the LSD method to exactly the same data. No crossover effect was detected for any star in the sample. Only for HD 98922 the crossover effect was found tobe close to 3σ with a measured value of –4228 ± 1443 km s^–1 G. A quadratic magnetic field of the order of 10 kG was detected in HD 98922, and of ∼3.5 kG in HD 104237. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An infrared study of Herbig Ae/Be stars

The IRAS and 2MASS associations for 206 HAEBE stars are identified in this paper. From the color-color diagrams and spectral index, it is found that the IR excesses for most samples are due to thermal emission from the circumstellar material. It is also found that the IR excesses at IRAS region for few HAEBE stars and the near-IR excesses for some HAEBE stars are likely attributed to free-free emission or free-bound emission from the circumstellar ionized gas. Moreover, the evolution scenario (from embedded HAEBE stars to β Pictoris-like main-sequence stars) from Malfait et al. (1998) has been checked, our result does not support this evolution scenario.This publication makes use of data products from the Two Micron All Sky Survey (2MASS), which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, founded by the National Aeronautics and Space Administration and the National Science Foundation of the United States.     

Probing the circumstellar structures of T Tauri stars and their relationship to those of Herbig stars

We present Hα spectropolarimetry observations of a sample of 10 bright T Tauri stars, supplemented with new Herbig Ae/Be star data. A change in the linear polarization across Hα is detected in most of the T Tauri (9/10) and Herbig Ae (9/11) objects, which we interpret in terms of a compact source of line photons that is scattered off a rotating accretion disc. We find consistency between the position angle (PA) of the polarization and those of imaged disc PAs from infrared and millimetre imaging and interferometry studies, probing much larger scales. For the Herbig Ae stars AB Aur, MWC 480 and CQ Tau, we find the polarization PA to be perpendicular to the imaged disc, which is expected for single scattering. On the other hand, the polarization PA aligns with the outer disc PA for the T Tauri stars DR Tau and SU Aur and FU Ori, conforming to the case of multiple scattering. This difference can be explained if the inner discs of Herbig Ae stars are optically thin, whilst those around our T Tauri stars and FU Ori are optically thick. Furthermore, we develop a novel technique that combines known inclination angles and our recent Monte Carlo models to constrain the inner rim sizes of SU Aur, GW Ori, AB Aur and CQ Tau. Finally, we consider the connection of the inner disc structure with the orientation of the magnetic field in the foreground interstellar medium: for FU Ori and DR Tau, we infer an alignment of the stellar axis and the larger magnetic field direction.     

Probing the circumstellar structure of Herbig Ae/Be stars

We present Hα spectropolarimetry observations of a sample of 23 Herbig Ae/Be stars. A change in the linear polarization across Hα is detected in a large fraction of the objects, which indicates that the regions around Herbig stars are flattened (disc-like) on small scales. A second outcome of our study is that the spectropolarimetric signatures for the Ae stars differ from those of the Herbig Be stars, with characteristics changing from depolarization across Hα in the Herbig Be stars, to line polarizations in the Ae group. The frequency of depolarizations detected in the Herbig Be stars (seven out of 12) is particularly interesting as, by analogy with classical Be stars, it may be the best evidence to date that the higher-mass Herbig stars are surrounded by flattened structures. For the Herbig Ae stars, nine out of 11 show a line polarization effect that can be understood in terms of a compact Hα emission that is itself polarized by a rotating disc-like circumstellar medium. The spectropolarimetric difference between the Herbig Be and Ae stars may be the first indication that there is a transition in the Hertzsprung–Russell diagram from magnetic accretion at spectral type A to disc accretion at spectral type B. Alternatively, the interior polarized line emission apparent in the Ae stars may be masked in the Herbig Be stars owing to their higher levels of Hα emission.     

On the binarity of Herbig Ae/Be stars

We present high-resolution spectro-astrometry of a sample of 28 Herbig Ae/Be and three F-type pre-main-sequence stars. The spectro-astrometry, which is essentially the study of unresolved features in long-slit spectra, is shown from both empirical and simulated data to be capable of detecting binary companions that are fainter by up to 6 mag at separations larger than ∼0.1 arcsec. The nine targets that were previously known to be binary are all detected. In addition, we report the discovery of six new binaries and present five further possible binaries. The resulting binary fraction is 68 ± 11 per cent. This overall binary fraction is the largest reported for any observed sample of Herbig Ae/Be stars, presumably because of the exquisite sensitivity of spectro-astrometry for detecting binary systems. The data hint that the binary frequency of the Herbig Be stars is larger than that of the Herbig Ae stars. The Appendix presents model simulations to assess the capabilities of spectro-astrometry and reinforces the empirical findings. Most spectro-astrometric signatures in this sample of Herbig Ae/Be stars can be explained by the presence of a binary system. Two objects, HD 87643 and Z CMa, display evidence for asymmetric outflows. Finally, the position angles of the binary systems have been compared with available orientations of the circumprimary disc and these appear to be coplanar. The alignment between the circumprimary discs and the binary systems strongly suggests that the formation of binaries with intermediate-mass primaries is due to fragmentation as the alternative, stellar capture, does not naturally predict aligned discs. The alignment extends to the most massive B-type stars in our sample. This leads us to conclude that formation mechanisms that do result in massive stars, but predict random angles between the binaries and the circumprimary discs, such as stellar collisions, are also ruled out for the same reason.     

Periodic phenomena in Ae / Be Herbig stars light curves

Light curves of 78 Herbig Ae/Be stars were studied by digital analysis methods. Twenty-five thousand photoelectricUBVR observations of these stars were made by the authors in 1978–1990. A star was usually observed once a night. The stars were observed between 70 and 770 nights and on average each star was observed at more than 300 nights. The mean epoch for a star is 5 years. A morphologic classification of the light curves is suggested. The method of digital analysis are briefly described. To estimate the reliability of cyclic phenomena discovered in our study, a large number of them has been examined.     

Protoplanetary dust clouds in disks of Herbig Ae/Be stars

The very large brightness decrease of late-type Herbig Ae/Be stars is believed to be caused by obscuring dust clouds orbiting in the outer parts of their circumstellar disks. The distances of the dust clouds to the central stars have been estimated using the wavelength at maximum flux of the excess near-IR radiation, Wien's displacement law, and a formula derived by Rowan-Robinson (1980). The critical masses of these clouds were calculated employing Chandrasekhar's (1943) formula. The minimum size of the dust grains in the obscuring clouds was estimated using Aumannet al.'s (1984) formula they had applied to the star Lyr. However, it can be about ten times smaller if the dust grains are situated at the back of the cloud. The average size of these grains has been determined by assuming a size distribution similar to that in the asteroidal belt (Dohnanyi, 1969) and in the interstellar medium (Mathiset al., 1977). Their number density was determined by means of the extinction power of the dust cloud at theV pass-band. The results of our calculations show that above parameters are similar to those in our solar system. Therefore, we believe that most probably (a) the formation of planetesimals in the circumstellar disks of Herbig Ae/Be stars is on-going; and (b) the obscuring clouds will, in the long run, become planet-like objects.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

A new photometric study of Herbig Ae/Be stars in the infrared

In this paper we collected almost all HAeBe stars known so far (253 sources in total) to photometrically study their infrared properties. The 2MASS, WISE, IRAS and AKARI data are employed to make analyses. It is shown from several two-color diagrams that from 1 µm to 60 µm infrared radiations from circumstellar disks with the power law distribution play a very important role for infrared excesses which are much larger than that for ordinary Be stars. In the WISE two-color diagram, (W2–W3) vs. (W1–W2), some sources show thermal emissions probably due to dust surrounded and enhanced PAH features at 3.3 and 11.3 µm. In the wavelength longer than 60 µm infrared radiations are not so influenced by the circumstellar disk, but mainly from the ISM surrounded.     

Porous dust grains in the shells of Herbig Ae/Be stars

The transfer of polarized radiation in inhomogeneous circumstellar shells with a spheroidal spatial distribution of porous dust particles is computed. The grains are modeled by an MRN mixture of silicate and graphite particles. The optical properties of porous particles (considered separately in the Appendix) are computed by using effective medium theory and Mie theory. The following observational characteristics have been computed for WW Vul, a typical Herbig Ae star with Algol-like minima: the spectral energy distribution from the ultraviolet to the far infrared, the color-magnitude diagrams, the wavelength dependence of linear polarization, and the shell brightness distribution. The effect of grain porosity on the results is considered. It has been found that only moderate particle porosity (the volume fraction of matter is f ～0.5) can explain available observational data in terms of the approach used. Since radiation pressure must rapidly sweep submicron-sized grains out of the vicinity of Herbig Ae/Be stars, we briefly discuss how particle porosity can affect this process.     

HD 52721 - A close binary system among the Ae/Be Herbig stars

Photometric and high resolution spectroscopic data for the Herbig star HD 52721 obtained from October 2009 through October 2010 are presented. We confirm the cyclical character of the photometric variability of HD 52721 discovered previously by others. The variability shows up as minima in the light curve of this star which repeat with a period P = 0^d.805 and are typical of eclipsing binary systems. ASAS data covering an observation period from September 2003 through December 2009 show that two neighboring photometric minima differ from one another, and the actual orbital period of the system must be twice as high (P = 1^d.610). This is confirmed by the spectroscopic data. These show that the center of gravity of the emission Hα line and the profile of the HeI 6678 line with a distinct, bright circumstellar component have a clear correlation with a period of 1^d.610. To all appearances, HD 52721 is a close binary system consisting of two class B2 stars with quite similar parameters. We propose that the observed cyclical variations in the circumstellar parameters may be related to the existence of a global azimuthal inhomogeneity within the shell which rotates synchronously with the orbital motion of the system components.     

Detection of accreting circumstellar gas around weak emission-line Herbig Ae/Be stars

We present archival and recent IUE high dispersion spectra of late B stars which reveal the presence of accreting gas with velocities as high as 350 km s^–1, collisional ionization of the accreting gas to temperatures above the stellar T_eff, and column densities intermediate between those observed toward classical Herbig Ae/Be stars and the nearby proto-planetary system Pictoris. One of the stars, HD 176386, while lacking obvious optical signatures of youth, is a member of the R CrA star formation region, and with an inferred age of 2.8 Myr has not yet arrived on the zero-age main sequence (ZAMS). The other object, an isolated, field B star with pronounced IR excess due to warm, circumstellar dust, 51 Oph, exhibits only modest h emission. The combination of high velocity, accreting gas in systems with IR excesses due to circumstellar dust suggests that not only are these objects candidate proto-planetary systems, but that they may represent an extension to higher stellar masses of the weak-emission pre-main sequence (PMS) stars.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Measuring the mass accretion rates of Herbig Ae/Be stars with X‐shooter

We present the results of our observations of eight magnetic Herbig Ae/Be stars obtained with the X‐shooter spectrograph mounted on UT2 at the VLT. X‐shooter provides a simultaneous, medium‐resolution and high‐sensitivity spectrum over the entire wavelength range from 300 to 2500 nm. We estimate the mass accretion rates (Ṁ_acc) of the targets from 13 different spectral diagnostics using empiric calibrations derived previously for T Tauri‐type stars and brown dwarfs. We have estimated the mass accretion rates of our targets, which range from 2 × 10^–9 to 2 × 10^–7 M_⊙ yr^–1. Furthermore, we have found accretion rate variability with amplitudes of 0.10–0.40 dex taking place on time scales from one day to tens of days. Additional future night‐to‐night observations need to be carried out to investigate the character of Ṁ_acc variability in details. Our study shows that the majority of the calibration relations can be applied to Herbig Ae/Be stars, but several of them need to be re‐calibrated on the basis of new spectral data for a larger number of Herbig Ae/Be stars (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dust around young stars. Model of envelope of the Ae Herbig star WW Vul

Leningrad State University; Crimean Astrophysical Observatory. Translated from Astrofizika, Vol. 34, No. 2, pp. 181–198, March–April, 1991.     

X‐shooting Herbig Ae/Be stars: Accretion probed by near‐infrared He I emission

The Herbig Ae/Be stars are intermediate mass pre‐main sequence stars that bridge the gap between the low mass T Tauri stars and the Massive Young Stellar Objects. In this mass range, the acting star forming mechanism switches from magnetically controlled accretion to an as yet unknown mechanism, but which is likely to be direct disk accretion onto the star. We observed a large sample of Herbig Ae/Be stars with X‐shooter to address this issue from a multi‐wavelength perspective. It is the largest such study to date, not only because of the number of objects involved, but also because of the large wavelength coverage from the blue to the near‐infrared. This allows many accretion diagnostics to be studied simultaneously. By correlating the various properties with mass, temperature and age, we aim to determine where and whether the magnetically controlled mass accretion mechanism halts and the proposed direct disk accretion takes over. Here, we will give an overview of the background, present some observations and discuss our initial results. We will introduce a new accretion diagnostic for the research of Herbig Ae/Be stars, the HeI 1.083 μm line (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)