Linear spectropolarimetry across the optical spectrum of Herbig Ae/Be stars

Abstract

We present the results of spectropolarimetric observations of 12 Herbig Ae/Be objects. Our data have the largest spectropolarimetric wavelength coverage, 4560Å to 9480Å, published to date. A change in linear polarisation across the Hα line, is detected in all objects. Such a line effect reveals the fact that stellar photons are scattered off free electrons that are not distributed in a spherically symmetric volume, suggesting the presence of small disks around these accreting objects. Thanks to the large wavelength coverage, we can report that Hα is the spectral line in the optical wavelength range that is most sensitive to revealing deviations from spherical symmetry, and the one most likely to show a line effect across the polarisation in such cases. Few other spectral lines display changes in polarisation across the line. In addition, Hα is the only line which shows an effect across its absorption component in some sources. We present a scenario explaining this finding and demonstrate that the detection of the line effect strongly relies on the number of photons scattered into our line of sight. We highlight the special case of R Mon, which is the only object in our sample to show many lines with a polarisation effect, which is much stronger than in all other objects. Given that the object and its nebulosity is spatially resolved, we argue that this is due to scattering of the stellar and emission spectrum off circumstellar dust.

A Decade of Linear and Circular Polarimetry with the POLISH2 Polarimeter

“…Our polarization results are generally consistent with the catalogs referenced in Tables 16 and 17 (Michalsky et al 1976;Schmidt et al 1992;Heiles 2000;Weitenbeck 2004;Elias et al 2008;Bailey et al 2010Bailey et al , 2020Ababakr et al 2016;Cotton et al 2017). However, Figure 16 shows that we measure the degree of linear polarization for HD 18537 and HD 97991 to be more than an order of magnitude lower than the values in the Heiles (2000) catalog.…”

A statistical spectropolarimetric study of Herbig Ae/Be stars

“…The polarization ranges from ∼0.3% to ∼2.0% with an average of ∼0.9 %. Only R Mon shows a magnitude of ∼10% which is not expected from electron scattering close to the star and is due to observational effects as the object is spatially resolved in these observations (see the discussion in Ababakr et al 2016). We have therefore discarded R Mon from the final results.…”

“…We combined all our previous spectropolarimetric work across Hα line of HAeBe stars (Oudmaijer & Drew 1999;Vink et al 2002Mottram et al 2007;Wheelwright et al 2011) into one sample. Our previous medium resolution linear spectropolarimetric data were obtained using the RGO spectrograph on the 3.9-m Anglo Australian Telescope (AAT) (Oudmaijer & Drew 1999), the ISIS spectrograph on the 4.2-m on the William Herschel Telescope (WHT), La Palma (Oudmaijer & Drew 1999;Vink et al 2002Mottram et al 2007;Wheelwright et al 2011;Ababakr et al 2016) and the FORS2 spectrograph mounted on ESO's 8.2-m Very Large Telescope (VLT) in Chile (Ababakr et al 2016). These bring the total number of observed HAeBe objects to 56 (31 HBe and 25 HAe).…”

“…2 also shows the intrinsic polarisation angle which is measured from the slope of the line effect in the (Q, U) diagram. For the depolarisation line effect, the angle is measured from the (unpolarized) line to the (polarized) continuum while for the intrinsic polarisation and McLean effect it is measured from continuum to (polarized) line (see the discussion in Ababakr et al 2016). Although the direction for both the depolarization (HD 240010) and intrinsic polarization (HD 163296) are the same in the respective QU graphs, the intrinsic polarization angles differ, as this angle is measured from the line center to the continuum in the case of the depolarization, it is measured in the opposite direction for the intrinsic polarization.…”

Medium-resolution near-infrared spectroscopy of massive young stellar objects

“…In this context it may be useful to note that earlier in its evolution, the star may be swollen due to the large accretion ratess (Hosokawa, Yorke & Omukai 2010) and generate a magnetic field at this stage (Hoare & Franco 2007). Current evidence for pre-Main Sequence Herbig Ae/Be stars suggests the transition in disk accretion mechanism occurs around the A-B spectral type boundary (Mottram et al 2007;Ababakr, Oudmaijer & Vink 2016). One of the few models put forward to explain the accretion mechanism in massive Herbig Be stars is the boundary layer where the circumstellar disk extends to the surface of the central star.…”