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Post Info TOPIC: Herbig Ae/Be stars
Blobrana


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RE: Herbig Ae/Be stars
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Title: Accretion rates and accretion tracers of Herbig Ae/Be stars
Authors: I. Mendigutía, N. Calvet, B. Montesinos, A. Mora, J. Muzerolle, C. Eiroa, R.D. Oudmaijer, B. Merín

This work aims to derive accretion rates for a sample of 38 HAeBe stars. We apply magnetospheric accretion (MA) shock modelling to reproduce the observed Balmer excesses. We look for possible correlations with the strength of the Halpha, [OI]6300, and Brgamma emission lines.
The median mass accretion rate is 2 x 10^-7 Msun yr^-1 in our sample. The model fails to reproduce the large Balmer excesses shown by the four hottest stars (T* > 12000 K). We derive Macc propto M*^5 and Lacc propto L*^1.2 for our sample, with scatter. Empirical calibrations relating the accretion and the Halpha, [OI]6300, and Brgamma luminosities are provided. The slopes in our expressions are slightly shallower than those for lower mass stars, but the difference is within the uncertainties, except for the [OI]6300 line. The Halpha 10% width is uncorrelated with Macc, unlike for the lower mass regime. The mean Halpha width shows higher values as the projected rotational velocities of HAe stars increase, which agrees with MA. The accretion rate variations in the sample are typically lower than 0.5 dex on timescales of days to months, Our data suggest that the changes in the Balmer excess are uncorrelated to the simultaneous changes of the line luminosities.
The Balmer excesses and Halpha line widths of HAe stars can be interpreted within the context of MA, which is not the case for several HBes. The steep trend relating Macc and M* can be explained from the mass-age distribution characterising HAeBe stars. The line luminosities used for low-mass objects are also valid to estimate typical accretion rates for the intermediate-mass regime under similar empirical expressions. However, we suggest that several of these calibrations are driven by the stellar luminosity.

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Blobrana


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Posts: 131433
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Herbig Ae protoplanetary disks
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Title: Hot and cool water in Herbig Ae protoplanetary disks. A challenge for Herschel
Authors: Peter Woitke, Wing-Fai Thi, Inga Kamp, Michiel R. Hogerheijde
(Version v2)

The spatial origin and detectability of rotational H2O emission lines from Herbig Ae type protoplanetary disks beyond 70 micron is discussed. We use the recently developed disk code ProDiMo to calculate the thermo-chemical structure of a Herbig Ae type disk and apply the non-LTE line radiative transfer code Ratran to predict water line profiles and intensity maps. The model shows three spatially distinct regions in the disk where water concentrations are high, related to different chemical pathways to form the water: (1) a big water reservoir in the deep midplane behind the inner rim, (2) a belt of cold water around the distant icy midplane beyond the snow-line r>20AU, and (3) a layer of irradiated hot water at high altitudes z/r=0.1...0.3, extending from about 1AU to 30AU, where the kinetic gas temperature ranges from 200K to 1500K. Although region 3 contains only little amounts of water vapour (~3x10^-5 M_Earth), we find this warm layer to be almost entirely responsible for the rotational water emission lines, except for the 3 lowest excitation lines. Thus, Herschel will probe first and foremost the conditions and radial extension of region 3, where water is predominantly formed via neutral-neutral reactions and the gas is thermally decoupled from the dust T_gas>T_dust. The observations do not allow for a determination of the snow-line, because the snow-line truncates the radial extension of region 1, whereas the lines originate from region 3. Different line transfer approximations (LTE, escape probability, Monte Carlo) are discussed. A non-LTE treatment is required in most cases, and the results obtained with the escape probability method are found to underestimate the Monte Carlo results by 2%...45%.

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Blobrana


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Posts: 131433
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RE: Herbig Ae/Be stars
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The discovery of magnetic "fossils" around young stars in the Milky Way has boosted the case for the existence of magnetic fields right after the big bang.
If primordial magnetic fields existed, they would have influenced how the universe evolved. For instance, they could have skewed its expansion in one direction.
Now, Claude Catala of the Paris Observatory in France and colleagues believe they have found "fossils" of primordial magnetic fields. The team were attempting to answer a puzzle in astronomy: why a small fraction of so-called main sequence A/B stars have very strong and ordered magnetic fields. These could have formed via two methods: a dynamo mechanism, due to their rotation, or by inheriting fossil fields that existed in the interstellar medium, the gas and dust between stars.

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Blobrana


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Title: Searching for a link between the magnetic nature and other observed properties of Herbig Ae/Be stars
Authors: S. Hubrig, C. Grady, M. Schoeller, 0. Schuetz, B. Stelzer, M. Pogodin, M. Cure, R. Yudin

We present the results of a new magnetic field survey of Herbig Ae/Be and A debris disk stars. They are used to determine whether magnetic field properties in these stars are correlated with the mass-accretion rate, disk inclinations, companion(s), Silicates, PAHs, or show a more general correlation with age and X-ray emission as expected for the decay of a remnant dynamo.

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Blobrana


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Posts: 131433
Date:
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The discovery of magnetic "fossils" around young stars in the Milky Way has boosted the case for the existence of magnetic fields right after the big bang.
If primordial magnetic fields existed, they would have influenced how the universe evolved. For instance, they could have skewed its expansion in one direction.
Now, Claude Catala of the Paris Observatory in France and colleagues believe they have found "fossils" of primordial magnetic fields. The team were attempting to answer a puzzle in astronomy: why a small fraction of so-called main sequence A/B stars have very strong and ordered magnetic fields. These could have formed via two methods: a dynamo mechanism, due to their rotation, or by inheriting fossil fields that existed in the interstellar medium, the gas and dust between stars.

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Blobrana


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Title: Magnetism of Herbig Ae/Be stars
Authors: G.A. Wade, E. Alecian, J. Grunhut, C. Catala, S. Bagnulo, C.P. Folsom, J.D. Landstreet

Observations of magnetic fields of stars at the pre-main sequence phase can provide important new insights into the complex physics of the late stages of star formation. This is especially true at intermediate stellar masses, where magnetic fields are strong and globally organised, and therefore most amenable to direct study. Recent circularly-polarised spectroscopic observations of pre-main sequence Herbig Ae/Be stars have revealed the presence of organised magnetic fields in the photospheres of a small fraction of these objects. To date, 9 magnetic HAeBe stars have been detected, and those detections confirmed by repeated observations. The morphology and variability of their Stokes V signatures indicates that their magnetic fields have important dipole components of kG strength, and that the dipole is stable on timescales of at least years. These magnetic stars exhibit a large range of stellar mass, from about 2-13 solar masses, and diverse rotational properties, with vsini from a few km/s to 200 km/s. Most magnetic HAeBe stars show approximately solar abundances; they clearly do not generally exhibit the strong and systematic peculiarities of the magnetic main sequence A and B type stars (the Ap/Bp stars). The observed fractional bulk incidence of magnetic HAeBe stars is about 7%, a value compatible with the incidence of magnetic intermediate-mass stars on the main sequence. This low incidence is at odds with formation scenarios generally involving magnetically-mediated accretion. The similarity between the magnetic properties of the pre-main sequence and main sequence intermediate-mass stars appears compatible with the hypothesis of a fossil origin of magnetism in these objects.

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Blobrana


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Title: Circumstellar disks around Herbig Be stars
Authors: T. Alonso-Albi, A. Fuente, R. Bachiller, R. Neri, P. Planesas, L. Testi, O. Berne, C. Joblin
(Version v2)

We have carried out a search for circumstellar disks around Herbig Be stars using the NRAO Very Large Array (VLA) and the IRAM Plateau de Bure (PdB) interferometers. In this Paper, we present our new VLA and PdBI data on the three objects MWC 297, Z CMa and LKHa 215. We have constructed the SED from near-IR to centimetre wavelengths by adding our millimetre and centimetre data to the available data at other wavelengths, mainly Spitzer images. The whole SED has been fitted using a disk+envelope model. In addition, we have compiled all the disk millimetre observations in the literature and made some statistics. We show that the disk mass is usually only a small percentage (less than 10%) of the mass of the whole envelope in HBe stars. Concerning the disks, there are large source to source variations. Two disks of our sample, R Mon and Z CMa, have similar sizes and masses to those found in T Tauri and Herbig Ae stars. The disks around MWC 1080 and MWC 297 are, however, smaller (rout<100 AU). We have not detected at millimetre wavelengths the disks towards MWC 137 and LkHa 215 which implies an upper limit to the mass and the size of the possible circumstellar disks. The comparison between our data and previous results in T Tauri and Herbig Ae stars shows that although massive disks (0.1 Msun) are found in very young objects (10^4 yr), the masses of the disks around Herbig Be stars are usually 5-10 times lower than those around lower mass stars. We propose that disk photo-evaporation is the responsible for this behaviour. In Herbig Be stars the UV radiation disperses the gas of the outer disk on a time-scale of a few 10^5 yr. Once the outer part of the disk is gone, the entire gaseous disk is photo-evaporated in a very short time-scale (10^5 yr) and only a small dusty disk composed of large grains remains.

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Blobrana


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An international team of astronomers led by E. Tatulli (Grenoble, France) and S. Kraus (Bonn, Germany)  used the unique capability of the VLT near-infrared interferometer, coupled with spectroscopy, to probe the gaseous environment of Herbig Ae/Be stars. These are young stars of intermediate mass (approximately 2 to 10 solar masses), which are still contracting and often show strong line emissions.
 
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Blobrana


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New high-resolution observations with the VLT Interferometer of the European Southern Observatory in Chile reveal gas infall and outflow processes in the direct environment of six young stars.

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Blobrana


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Young stellar objects: The source of gas emission around Herbig Ae/Be stars
This week, Astronomy & Astrophysics is publishing new observations with AMBER/VLTI of the gas component in the vicinity of young stars. An international team of astronomers led by E. Tatulli (Grenoble, France) and S. Kraus (Bonn, Germany) used the unique capability of the VLT near-infrared interferometer, coupled with spectroscopy, to probe the gaseous environment of Herbig Ae/Be stars. These are young stars of intermediate mass (approximately 2 to 10 solar masses), which are still contracting and often show strong line emissions.

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