HAeBe Stars Research Articles

A new photometric study of B[e] stars in the infrared

Abstract We have collected almost all B[e] stars (120 stars in total) listed in the literature up to the present time. We have then photometrically studied their infrared properties by using data from IRAS, WISE, and 2MASS. From several two-color diagrams we found that infrared radiation for the majority of B[e] stars shows a power law distribution from dusty/gaseous circumstellar disks in the 1–60 μm region. We also found that, possibly due to the influence of polycyclic aromatic hydrocarbon and silicate emissions, some B[e] stars have a blackbody distribution in the 3–12 μm region, and due to the influence of clouds/nebulae nearby some B[e] stars also show blackbody distributions in the IRAS wavelengths. Furthermore, we compared infrared properties between B[e] stars and Be stars, and between B[e] stars and HAeBe stars. We show that the infrared excess of B[e] stars is much larger than for Be stars in the 1–25 μm region, while both B[e] and HAeBe stars have similar infrared color properties in the 1–25 μm region.

Spectral study of V565 Mon: probable FU Ori-like or chemically peculiar star

We present a detailed spectroscopic study of pre-main sequence star V565 Mon, which is the illuminating star of the Parsamian 17 cometary nebula. Observations were performed with the 2.6 m telescope in Byurakan Astrophysical Observatory on 2018 February 15. Radial velocities and equivalent widths of the most prominent lines of V565 Mon are presented. We build the spectral energy distribution and estimate the main parameters of the star, for example the obtained bolometric luminosity of V565 Mon is L V565 ≈ 130 L ⊙. Considering all features of V565 Mon, we come to the conclusion that this young intermediate-mass star belongs to some intermediate class between T Tau and HAeBe stars. Very unusual for a young star is the presence of strong absorption Ba II lines in the spectrum. Possible explanations on this issue are discussed. Hence, we think that V565 Mon is a unique example, which can help to understand some open questions involved in the problem of nucleosynthesis in young stars.

Spectroscopic analysis of accretion/ejection signatures in the Herbig Ae/Be stars HD 261941 and V590 Mon

ABSTRACT Herbig Ae/Be (HAeBe) stars are the intermediate-mass analogues of low-mass T Tauri stars. Both groups may present signs of accretion, outflow, and IR excess related to the presence of circumstellar discs. Magnetospheric accretion models are generally used to describe accreting T Tauri stars, which are known to have magnetic fields strong enough to truncate their inner discs and form accretion funnels. Since few HAeBe stars have had magnetic fields detected, they may accrete through a different mechanism. Our goal is to analyse the morphology and variability of emission lines that are formed in the circumstellar environment of HAeBe stars and use them as tools to understand the physics of the accretion/ejection processes in these systems. We analyse high-resolution (R ∼ 47 000) UVES/ESO spectra of two HAeBe stars – HD 261941 (HAe) and V590 Mon (HBe) that are members of the young (∼3 Myr) NGC 2264 stellar cluster and present indications of sufficient circumstellar material for accretion and ejection processes to occur. We determine stellar parameters with synthetic spectra, and also analyse and classify circumstellar lines such as H α, H β, and He i λ5875.7, according to their morphologies. We model the H α mean line profile, using a hybrid Magnetohydrodynamics (MHD) model that includes a stellar magnetosphere and a disc wind, and find signatures of magnetically driven outflow and accretion in HD 261941, while the H α line of V590Mon seems to originate predominantly in a disc wind.

Carbonaceous nano-dust emission in proto-planetary discs: the aliphatic-aromatic components

Context. In the interstellar medium, carbon (nano-)grains are a major component of interstellar dust. This solid phase is more vulnerable to processing and destruction than its silicate counterpart. It exhibits a complex, size-dependent evolution that is due to interactions within different radiative and dynamical environments. Infrared signatures of these carbon nano-grains are seen in a large number of discs around Herbig HAeBe stars. Aims. We probe the composition and evolution of carbon nano-grains at the surface of (pre-)transitional proto-planetary discs around Herbig stars. Methods. We present spatially resolved infrared emission spectra obtained with the Nasmyth Adaptive Optics System (NAOS) Near-Infrared Imager and Spectrograph (CONICA) at the Very Large Telescope (VLT) in the 3–4 μm range with a spatial resolution of 0.1′′, which allowed us to trace aromatic, olefinic, and aliphatic bands that are attributed to sub-nanometer hydrocarbon grains. We applied a Gaussian fitting to analyse the observed spectral signatures. Finally, we propose an interpretation in the framework of the The Heterogeneous dust Evolution Model of Interstellar Solids (THEMIS). Results. We show the presence of several spatially extended spectral features that are related to aromatic and aliphatic hydrocarbon material in discs around Herbig stars, from ~10 to 50–100 au, and even in inner gaps that are devoid of large grains. The correlation and constant intensity ratios between aliphatic and aromatic CH stretching bands suggests a common nature of the carriers. Given their expected high destruction rates through UV photons, our observations suggest that they are continuously replenished at the disc surfaces.

HERBIG Ae/Be CANDIDATE STARS IN THE INNERMOST GALACTIC DISK: QUARTET CLUSTER

ABSTRACT In order to investigate the Galactic-scale environmental effects on the evolution of protoplanetary disks, we explored the near-infrared (NIR) disk fraction of the Quartet cluster, which is a young cluster in the innermost Galactic disk at the Galactocentric radius R g ∼ 4 kpc ?> . Because this cluster has a typical cluster mass of ∼103 M ⊙ ?> as opposed to very massive clusters, which have been observed in previous studies (>104 M ⊙ ?> ), we can avoid intra-cluster effects such as strong UV field from OB stars. Although the age of the Quartet is previously estimated to be 3–8 Myr old, we find that it is most likely ∼3–4.5 Myr old. In moderately deep JHK images from the United Kingdom Infrared Telescope Infrared Deep Sky Survey, we found eight HAeBe candidates in the cluster, and performed K-band medium-resolution ( R ≡ Δ λ / λ ∼ 800 ?> ) spectroscopy for three of them with the Subaru 8.2 m telescope. These are found to have both Brγ absorption lines as well as CO bandhead emission, suggesting that they are HAeBe stars with protoplanetary disks. We estimated the intermediate-mass disk fraction (IMDF) to be ∼25% for the cluster, suggesting slightly higher IMDF compared to those for young clusters in the solar neighborhood with similar cluster age, although such a conclusion should await future spectroscopic study of all candidates of cluster members.

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.

The masses of young stars: CN as a probe of dynamical masses

Aims: We attempt to determine the masses of single or multiple young T Tauri and HAeBe stars from the rotation of their Keplerian disks. Methods:We used the IRAM PdBI interferometer to perform arcsecond resolution images of the CN N=2-1 transition with good spectral resolution. Integrated spectra from the 30-m radiotelescope show that CN is relatively unaffected by contamination from the molecular clouds. Our sample includes 12 sources, among which isolated stars like DM Tau and MWC 480 are used to demonstrate the method and its accuracy. We derive the dynamical mass by fitting a disk model to the emission, a process giving M/D the mass to distance ratio. We also compare the CN results with higher resolution CO data, that are however affected by contamination. Results: All disks are found in nearly perfect Keplerian rotation. We determine accurate masses for 11 stars, in the mass range 0.5 to 1.9 solar masses. The remaining one, DG Tau B, is a deeply embedded object for which CN emission partially arises from the outflow. With previous determination, this leads to 14 (single) stars with dynamical masses. Comparison with evolutionary tracks, in a distance independent modified HR diagram, show good overall agreement (with one exception, CW Tau), and indicate that measurement of effective temperatures are the limiting factor. The lack of low mass stars in the sample does not allow to distinguish between alternate tracks.

The galactic unclassified B[e] star HD 50138

The observed spectral variation of HD 50138 has led different authors to classify it in a very wide range of spectral types and luminosity classes (from B5 to A0 and III to Ia) and at different evolutionary stages as either HAeBe star or classical Be. Aims: Based on new high-resolution optical spectroscopic data from 1999 and 2007 associated to a photometric analysis, the aim of this work is to provide a deep spectroscopic description and a new set of parameters for this unclassified southern B[e] star and its interstellar extinction. Methods: From our high-resolution optical spectroscopic data separated by 8 years, we perform a detailed spectral description, presenting the variations seen and discussing their possible origin. We derive the interstellar extinction to HD 50138 by taking the influences of the circumstellar matter in the form of dust and an ionized disk into account. Based on photometric data from the literature and the new Hipparcos distance, we obtain a revised set of parameters for HD 50138. Results: Because of the spectral changes, we tentatively suggest that a new shell phase could have taken place prior to our observations in 2007. We find a color excess value of E(B-V) = 0.08 mag, and from the photometric analysis, we suggest that HD 50138 is a B6-7 III-V star. A discussion of the different evolutionary scenarios is also provided.

Spectropolarimetry of the Hα Line in Herbig Ae/Be Stars

Using the HiVIS spectropolarimeter built for the Haleakala 3.7 m AEOS telescope, we have obtained a large number of high-precision spectropolarimetrc observations (284) of Herbig AeBe stars collected over 53 nights totaling more than 300 hr of observing. Our sample of five HAeBe stars, AB Aurigae, MWC 480, MWC 120, MWC 158, and HD 58647, all show systematic variations in the linear polarization amplitude and direction as a function of time and wavelength near the Hα line. In all our stars, the Hα line profiles show evidence of an intervening disk or outflowing wind, evidenced by strong emission with an absorptive component. The linear polarization varies by 0.2%-1.5% with the change typically centered in the absorptive part of the line profile. These observations are inconsistent with a simple disk-scattering model or a depolarization model that produce polarization changes centered on the emissive core. We speculate that polarized absorption via optical pumping of the intervening gas may be the cause.

Sub-arcsec imaging of the AB Aur molecular disk and envelope at millimeter wavelengths: a non Keplerian disk

We present sub-arcsecond images of AB Auriga obtained with the IRAM Plateau de Bure interferometer in the isotopologues of CO, and in continuum at 3 and 1.3 mm. These observations allow us to trace the structure of the circumstellar material of AB Aur in regions where optical and IR imaging is impossible because of the emission from the star. These images reveal that the environment of AB Aur is widely different from the proto-planetary disks that surround T Tauri stars like DM Tau and LkCa15 or HAeBe stars like MWC 480 in several aspects. Instead of being centrally peaked, the continuum emission is dominated by a bright, asymmetric (spiral-like) feature at about 140 AU from the central star. Little emission is associated with the star itself. The molecular emission shows that AB Aur is surrounded by a very extended flattened structure (“disk”), which is rotating around the star. Bright molecular emission is also found towards the continuum “spiral”. The large-scale molecular structure suggests the AB Aur disk is inclined between 23 and 43 degrees, but the strong asymmetry of the continuum and molecular emission prevents an accurate determination of the inclination of the inner parts. Analysis of the emission in terms of a Keplerian disk provides a reasonable fit to the data, but fails to give a consistent picture because the inclinations determined from , , and do not agree. The mass predicted for the central star in such Keplerian models is in the range 0.9 to 1.2 , much smaller than the expected 2.2 from the spectral type of AB Aur. Better and more consistent fits to the , data are obtained by relaxing the Keplerian hypothesis. We find significant non-Keplerian motion, with a best fit exponent for the rotation velocity law of 0.41 ± 0.01, but no evidence for radial motion. The disk has an inner hole about 70 AU in radius. The disk is warm and shows no evidence of depletion of CO. The dust properties suggest that the dust is less evolved than in typical T Tauri disks. Both the spiral-like feature and the departure from purely Keplerian motion indicates the AB Aur disk is not in quasi-equilibrium. Disk self-gravity is insufficient to create the perturbation. This behavior may be related either to an early phase of star formation in which the Keplerian regime is not yet fully established and/or to a disturbance of yet unknown origin. An alternate, but unproven, possibility is that of a low mass companion located about 40 AU from AB Aur.

IX Ophiuchi: A High-Velocity Star Near a Molecular Cloud

The molecular cloud Barnard 59 is probably an outlier of the Upper Sco/ρ Oph complex. B59 contains several T Tauri stars (TTSs), but outside its northwestern edge are three other Hα-emission objects whose nature has been unclear: IX, KK, and V359 Oph. This paper is a discussion of all three and of a nearby Be star (HD 154851), based largely on Keck HIRES spectrograms obtained in 2004. KK Oph is a close (16) double. The brighter component is an HAeBe star, and the fainter is a K-type TTS. The complex BVR variations of the unresolved pair require both components to be variable. V359 Oph is a conventional TTS. Thus, these pre-main-sequence stars continue to be recognizable as such well outside the boundary of their parent cloud. IX Oph is quite different. Its absorption spectrum is about type G, with many peculiarities: all lines are narrow but abnormally weak, with structures that depend on ion and excitation level and that vary in detail from month to month. It could be a spectroscopic binary of small amplitude. Hα and Hβ are the only prominent emission lines. They are broad, with variable central reversals. However, the most unusual characteristic of IX Oph is the very high (heliocentric) radial velocity: about -310 km s-1, common to all spectrograms, and very different from the radial velocity of B59, about -7 km s-1. There is no detectable Li I λ6707 line. There is reason to believe that IX Oph is actually a background object, only aligned with B59. Several conceivable interpretations are discussed: (1) It is unlikely that it is a high-velocity ejectee from the Upper Sco or Upper Cen-Lup associations (the lack of detectable λ6707 shows that it is not the product of a very recent event, and the proper motion points in the wrong direction) or that it was born in or ejected from one of the distant high-velocity CO clouds at this longitude (l = 357°). (2) A stronger possibility is that it is simply a metal-poor high-velocity G- or K-type giant (but such stars are not irregularly variable in light and do not have such strong Balmer emission lines). More likely, (3) IX Oph is a member of the high-velocity, low-metallicity SRd class of semiregular variables found in the field and in some globular clusters. At some phases, those stars show Hα emission like that found in IX Oph and, in one example, emission lines of neutral metals and double absorption lines as in IX Oph.

Seeking the progenitors of magnetic Ap stars: A search for magnetic fields in HAeBe stars using FORS1 and ESPaDOnS

We report the discovery, using FORS1 at the ESO-VLT and ESPaDOnS at the Canada-France-Hawaii Telescope, of magnetic fields and chemical peculiarities in the Herbig Ae/Be stars HD 72106 and HD 101412. These stars may well represent pre-main sequence progenitors of the magnetic Ap/Bp stars. At the same time, we fail to confirm claims by Hubrig et al. (2004) of the presence of magnetic fields in the Herbig Ae star HD 139614.

Spectral and Photoelectric Studies of the Herbig Ae/Be Star HD 259431

Spectral and photoelectric (ubvy, Hα, Hβ) observations of the Herbig Ae/Be star HD 259431 are reported. It is found that as its brightness fades, this star becomes bluer in the Paschen continuum and the intensity and equivalent width of the hydrogen emission lines increase. The spectral observations reveal significant variations in the intensity of the Mg II 4481 A photospheric absorption line. A rise and fall in the luminosity by 0m.04 within a period of 5-7 minutes was recorded. Radical variations in the Hα lineshape ("double" → "P Cyg") and flare activity are not only observed in this star, but also in a number of HAEBE stars. It is suggested that flare activity may initiate a change in the velocity gradient at the base of the wind and, thereby, induce "double → P Cyg" or "P Cyg → single" transitions. The nonradial pulsations of this star are also discussed.

Herbig Ae/Be stars with the IRAS low-resolution spectra

IRAS low-resolution spectra are presented for 36 HAEBE stars. It is found that silicate dust is common material in the circumstellar disks or/and shells of HAEBE stars. It is also found that the PAH feature often appears as well. In addition, the correlation between the dust features and spectral type, and infrared excesses can be found for most HAEBE stars.

Wind and Disk Diagnostics in PMS Stars. A Critical Analysis

The relationships among traditional wind and disk diagnostics - Hα and [OI]λ6300 lines and IR luminosity excesses, respectively - and star parameters are critically analysed. The total sample includes 109 PMS stars - 20 Weak-line T Tauri (WTTS), 45 Classical T Tauri (CTTS) and 44 HAeBe stars-. Our results suggest that Hα is neither a wind nor an accretion tracer. Hα and [OI] emissions seem to correlate very well with the photospheric luminosity and not with ΔLIR/Lph, a parameter related to the origin of the IR excesses.