Structure of the inner regions of the circumstellar gas envelopes of young hot stars. I. The isolated Ae Herbig star WW Vul

Results of simultaneous spectral and photometric monitoring of the Ae Herbig star WW Vul in the neighborhoods of the Ha line and the sodium NaI D resonance doublet are reported. It is shown that the spectral variability of the star is caused mainly by the anisotropic disk wind, whose high velocity component forms in the inner region of the accretion disk. The circumstellar gas in footpoint of the wind shows the variability of the density and velocity, that is in good agreement with the results of modeling of an accretion and outflows around young stars controlled by the stellar and/or disk magnetic field. An analysis of the variability of the parameters of the Ha emission line also showed that the density of the gas in the inner region of the accretion disk varies over a time scale exceeding 10 years. __________ Translated from Astrofizika, Vol. 49, No. 2, pp. 171–185 (May 2006).     

Detection of optical circular polarization in the Herbig Ae star WW Vul

UBVRI observations of circular polarization in WW Vul are presented. A positive polarization of ～0.1% was detected with a signal-to-noise ratio from 3 to 5 in each of the bands and more than 5 when averaged over all five bands. This observed polarization roughly corresponds to a 1% circular polarization of the radiation scattered in a circumstellar disk, which is most likely attributable to the significant alignment of scattering nonspherical dust grains. Since grain alignment is possible only in a magnetic field, this result provides circumstantial evidence for the existence of a magnetic field in the circumstellar disk of WW Vul.     

Photoelectric Investigation of the Herbig Ae/Be Star MWC 1080

The results of photoelectric observations of the Herbig Ae/Be star MWC 1080 in the Strömgren system are given. It was found that MWC 1080 undergoes rapid rises and falls in brightness with an amplitude of up to 0 ^m .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.     

The Herbig Ae/Be Star V586 Ori

Studies of the Herbig Ae/Be star V586 Ori based on photoelectric observations in the Strömgren system are reported. It is found that as the brightness decreases, the Balmer discontinuity index initially increases and then begins to drop. As the star fades, the index decreases. We have previously found similar variations in the indices and C for UX Ori, RR Tau, and V351 Ori. It is shown that V586 Ori has the characteristics of a shell-star.     

The Unusual Herbig Ae/Be Star XY Per A

The results of an investigation of XY Per based on photoelectric observations in the Strömgren 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 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.     

Polarimetry of QQ Vul

This paper presents an investigation of the polarization behaviour of the AM Her system QQ Vul at several epochs over 3 years. There are changes of a factor of 2 in the amplitude of the orbitally phased circular polarization from epoch to epoch. The mean behaviour can be modelled with an accretion arc ahead of and closer to the orbital plane than the magnetic pole, but it has proven difficult to link the changes in polarization with changes in the location of the accretion region. The paper concludes with an investigation into whether the variations are periodic, but the evidence for this is marginal.     

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

Spectral and photoelectric (ubvy, H, H) 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 Å photospheric absorption line. A rise and fall in the luminosity by 0^m.04 within a period of 5-7 minutes was recorded. Radical variations in the H 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.     

CK Vul: reborn perhaps, but not hibernating

It has been claimed that CK Vul (the remnant of Nova Vul 1670) may be the oldest recovered 'old nova' and as such provides evidence in support of the hibernation scenario for classical nova systems. However this interpretation has been challenged. We present 450- and 850-μm photometry of CK Vul which cast further doubt on its old nova status. It displays a large far infrared-submillimetre flux excess, inconsistent with the properties of an old nova. Furthermore, IRAS images show that CK Vul is located in a 'cavity' in the infrared emission, a feature often associated with planetary nebulae. It seems more likely that CK Vul – and hence Nova Vul 1670 – is (like V605 Aql and V4334 Sgr) an evolved star in the throes of a final thermal pulse.     

Dynamical Processes in the Neighborhood of the Herbig Ae Star MWC 480 Based on Spectral Monitoring Data

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 5876 line, the oxygen O I 7774 line, the H line, and some others. The H 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 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.     

Hα spectropolarimetry of the Herbig Ae star AB Aurigae

We present spectropolarimetric observations, obtained at H α , of the Herbig Ae star AB Aurigae. Changes in linear polarization across the H α line probe structure in the immediate circumstellar environment of the central star, down to scales of the order of one to a few stellar radii. In the case of AB Aurigae the observed polarimetric signature is complex. After applying a correction for foreground continuum polarization, we find that there is a linear-polarized H α emission component intrinsic to the source. Rotation of the angle of polarization through the emission-line profile suggests scattering in a rotating circumstellar disc. The magnetic accretor model commonly applied to T Tauri stars shows promise of explaining these data.     

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.     

Pulsation in the pre-main-sequence Herbig Ae star HD 142666

Asteroseismology of pre-main-sequence δ Scuti stars has the potential not only to provide unprecedented constraints on models of these stars, but also to allow for the possibility of detecting evolutionary period changes, thus providing a direct measure of the pre-main-sequence evolutionary time-scale. In the last two years, the published number of such stars known has doubled from four to eight. Searches are now being conducted amongst the Herbig Ae stars, which are considered to be excellent candidates. We announce the discovery of δ Scuti pulsation in one Herbig Ae star, HD 142666, which lies within Marconi & Palla's theoretically predicted instability strip for pre-main-sequence stars, making this the ninth known pre-main-sequence δ Scuti star. We also demonstrate a lack of δ Scuti pulsation in another such star, HD 142527.     

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)     

Chemical spots on the surface of the strongly magnetic Herbig Ae star HD 101412

Due to the knowledge of the rotation period and the presence of a rather strong surface magnetic field, the sharp‐lined young Herbig Ae star HD 101412 with a rotation period of 42 d has become one of the most well‐studied targets among the Herbig Ae stars. High‐resolution HARPS polarimetric spectra of HD 101412 were recently obtained on seven different epochs. Our study of the spectral variability over the part of the rotation cycle covered by HARPS observations reveals that the line profiles of the elements Mg, Si, Ca, Ti, Cr, Mn, Fe, and Sr are clearly variable while He exhibits variability that is opposite to the behaviour of the other elements studied. Since classical Ap stars usually show a relationship between the magnetic field geometry and the distribution of element spots, we used in our magnetic field measurements different line samples belonging to the three elements with the most numerous spectral lines, Ti, Cr, and Fe. Over the time interval covered by the available spectra, the longitudinal magnetic field changes sign from negative to positive polarity. The distribution of field values obtained using Ti, Cr, and Fe lines is, however, completely different compared to the magnetic field values determined in previous low‐resolution FORS 2 measurements, where hydrogen Balmer lines are the main contributors to the magnetic field measurements, indicating the presence of concentration of the studied iron‐peak elements in the region of the magnetic equator. Further, we discuss the potential role of contamination by the surrounding warm circumstellar matter in the appearance of Zeeman features obtained using Ti lines. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Long-Term Spectral Variability of the Herbig Ae-Star HD 179218

The results of high-resolution long-term spectral monitoring of Herbig Ae star HD 179218 in the region of emission H line and the sodium resonance doublet Na I D are presented. The received data show the existence of short-term variability (about 10 days) and long-term wave-like variability of equivalent width EW, intensity of circumstellar (CS) emission I and parameters of H emission profile. The analysis of these data allows us to suppose the existence of the global variability of parameters of accretion disk.     

The exceptional Herbig Ae star HD 101412: The first detection of resolved magnetically split lines and the presence of chemical spots in a Herbig star

In our previous search for magnetic fields in Herbig Ae stars, we pointed out that HD 101412 possesses the strongest magnetic field among the Herbig Ae stars and hence is of special interest for follow‐up studies of magnetism among young pre‐main‐sequence stars. We obtained high‐resolution, high signal‐to‐noise UVES and a few lower quality HARPS spectra revealing the presence of resolved magnetically split lines. HD 101412 is the first Herbig Ae star for which the rotational Doppler effect was found to be small in comparison to the magnetic splitting and several spectral lines observed in unpolarized light at high dispersion are resolved into magnetically split components. The measured mean magnetic field modulus varies from 2.5 to 3.5kG, while the mean quadratic field was found to vary in the range of 3.5 to 4.8 kG. To determine the period of variations, we used radial velocity, equivalent width, line width, and line asymmetry measurements of variable spectral lines of several elements, as well as magnetic field measurements. The period determination was done using the Lomb‐Scargle method. The most pronounced variability was detected for spectral lines of He I and the iron peak elements, whereas the spectral lines of CNO elements are only slightly variable. From spectral variations and magnetic field measurements we derived a potential rotation period P_rot = 13.86 d, which has to be proven in future studies with a larger number of observations. It is the first time that the presence of element spots is detected on the surface of a Herbig Ae/Be star. Our previous study of Herbig Ae stars revealed a trend towards stronger magnetic fields for younger Herbig Ae stars, confirmed by statistical tests. This is in contrast to a few other (non‐statistical) studies claiming that magnetic Herbig Ae stars are progenitors of the magnetic Ap stars. New developments in MHD theory show that the measured magnetic field strengths are compatible with a current‐driven instability of toroidal fields generated by differential rotation in the stellar interior. This explanation for magnetic intermediate‐mass stars could be an alternative to a frozen‐in fossil field (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The absorption and emission spectrum of the magnetic Herbig Ae star HD 190073

We determine abundances from the absorption spectrum of the magnetic Herbig Ae star HD 190073 (V1295 Aql). The observations are primarily from HARPS spectra obtained at a single epoch. We accept arguments that the presence of numerous emission lines does not vitiate a classical abundance analysis, though it likely reduces the achievable accuracy. Most abundances are closely solar, but several elements show departures of a factor of two to three, as an earlier study has also shown. We present quantitative measurements of more than 60 emission lines, peak intensities, equivalent widths, and FWHM's. The latter range from over 200 km s^–1(Hα, He D₃) down to 10–20 km s^–1(forbidden lines). Metallic emission lines have intermediate widths. We eschew modeling, and content ourselves with a presentation of the observations a successful model must explain. Low‐excitation features such as the Na I D‐lines and [O I] appear with He I D₃, suggesting proximate regions with widely differing T_e and N_e as found in the solar chromosphere. The [O I] and [Ca II] lines show sharp, violet‐shifted features. Additionally, [Fe II] lines appear tobe weakly present in emission (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Rapid variability of the line He I λ 587.6 nm in the spectrum of the Herbig Ae/Be star WW Vul

We investigate the behavior of the relatively strong absorption line He I λ 587.6 nm in the spectrum of the rapid irregular variable star WW Vul (Sp A3) with the Algol-like brightness fadings. As many as 76 spectra of this star were acquired with the 2.6-m ZTSh telescope of the Crimean Astrophysical Observatory with the use of the medium-dispersion spectrograph SPEM. We show that the equivalent width of the He I line exhibits not only daily variations but also rapid oscillations with a typical period of several tens of minutes. Quasi-simultaneous observations of several standard stars with known energy distributions allowed us to calculate the monochromatic magnitudes of the variable at λ = 550 nm. There was no correlation between the equivalent widths of the line He I λ 587.6 nm and the monochromatic brightness of the star. At times we could identify the short-wave emission component of the He I line. We used the H_β profiles to confirm the spectral type A2–A3 of WW Vul.