Searching for Magnetospheres around Herbig Ae/Be Stars

We describe four different approaches for the detection of magnetospheric accretion among Herbig Ae/Be stars with accretion disks. Studies of several unique objects have been carried out. One of the objects is the Herbig Ae star HD 101412 with a comparatively strong magnetic field. The second is the early-type Herbig B6e star HD 259431. The existence of a magnetosphere in these objects was not recognized earlier. In both cases, a periodicity in the variation of some line parameters, originating near the region of the disk/star interaction, has been found. The third object is the young binary system HD 104237, hosting a Herbig Ae star and a T Tauri star. Based on the discovery of periodic variations of equivalent widths of atmospheric lines in the spectrum of the primary, we have concluded that the surface of the star is spotted. Comparing our result with an earlier one, we argue that these spots can be connected with the infall of material from the disk onto the stellar surface through a magnetosphere. The fourth example is the Herbig Ae/Be star HD 37806. Signatures of magnetospheric accretion in this object have been identified using a different method. They were inferred from the short-term variability of the He i λ5876 line profile forming in the region of the disk/star interaction.

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Photometric studies of Be star variability

Symposium - International Astronomical Union ◽

10.1017/s007418090021512x ◽

1994 ◽

Vol 162 ◽

pp. 299-300

Author(s):

John R. Percy

Keyword(s):

Time Scales ◽

Balmer Line ◽

Radial Pulsation ◽

Be Stars ◽

Photometric Study ◽

Short Term ◽

Hot Stars ◽

Be Star ◽

Short Term Variability

Be stars are hot stars which have shown emission in at least one Balmer line on at least one occasion. As the definition implies, the Be phenomenon can be variable with time: on time scales of days to decades as the circumstellar disc develops and disperses; on time scales of days to months in a few Be stars which are interacting binaries; on time scales of 0.2 to 2 days due to non-radial pulsation or possibly rotation. The Be stars are worthy of photometric study because they are bright and numerous; the nature of the short-term variability is not yet agreed upon; the cause of the development of the disc - and its relationship to the short-term variability - is also not yet known.

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Some Important Results From Two Photometric Campaigns on Short Term Variability in Be Stars

International Astronomical Union Colloquium ◽

10.1017/s0252921100116082 ◽

1987 ◽

Vol 92 ◽

pp. 90-92

Author(s):

C.R. Stagg

Keyword(s):

Be Stars ◽

Short Term ◽

Be Star ◽

Comparison Stars ◽

Set Up ◽

Short Term Variability

In 1983 and 1984 a programme of UBV observations was carried out to set up a network of standard stars, identify short term variables, and analyse the variability of the 86 bright (V ≳ 6.5) southern (δ < -20°) Be stars.The timing of the observations was designed specifically to reveal short term variability. Each group of Be and comparison stars was observed at least four times. Where possible, the first two sets of observations were made on one night and the next two sets on the next night (and sometimes a fifth set of observations was also made). Eleven Be stars were identified as possible short or intermediate term variables (Stagg, in preparation). These included the Be star 28 ω CMa, whose V and B-V (but not U-B) variations were consistent with the spectroscopic period of 1.37 days found by Baade (1982).

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Chinese and Japanese Participation in B/Be Star Campaigns

International Astronomical Union Colloquium ◽

10.1017/s0252921100056153 ◽

2000 ◽

Vol 175 ◽

pp. 364-371

Author(s):

J. Hao

Keyword(s):

Astronomical Observatory ◽

Be Stars ◽

Short Term ◽

Be Star ◽

Short Term Variability

AbstractIn the early 1980’s, astronomers at the Beijing Astronomical Observatory (BAO) entered a collaboration in a long term photometric observing project on Be stars which included about 30 stars. For example, in 1983 the group at BAO organized a multi-site campaign to measure the short term variability of EW Lac, KX And, KY And, o And, and LQ And. In 1989, 48 Per was selected to be one of the targets of MuSiCoS campaign in which many stellar astronomers at the BAO participated. In 1991, the Be group at the BAO joined a campaign on the prototypical nonradially pulsating B star, 53 Per. In 1992, French & Chinese astronomers proposed a joint campaign on o And and EW Lac. Astronomers in Japan are also active in Be star campaigns and in 1993-5 they organized several campaigns on few well known Be stars (ζ Oph, EW Lac, ϒ Cas, λ Eri and 28 Cyg).

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Short Term Photometric and Spectroscopic Variability of a Sample of Be Stars

International Astronomical Union Colloquium ◽

10.1017/s0252921100116033 ◽

1987 ◽

Vol 92 ◽

pp. 74-77

Author(s):

M. Alvarez ◽

R. Michel

Keyword(s):

Be Stars ◽

Short Term ◽

Preliminary Results ◽

Cooperative Program ◽

Short Term Variability

Previous studies of variable Be stars show that the short-term variability is a common, but not well understood phenomena. From a list of ‘candidates’ given by Alvarez (1980) and Schuster & Alvarez (1983) under a cooperative program between France and México, we choose some interesting Be objects to study their behaviour both in spectroscopy and photometry. We are reporting some preliminary results of our observations.

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Observations of magnetic fields in Herbig Ae/Be stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921319003764 ◽

2018 ◽

Vol 14 (A30) ◽

pp. 123-123

Author(s):

Markus Schöller ◽

Swetlana Hubrig

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

T Tauri Stars ◽

Be Stars ◽

T Tauri ◽

Weak Magnetic Fields ◽

Order Of Magnitude ◽

Field Geometry ◽

Herbig Ae Stars ◽

Magnetospheric Accretion

AbstractModels of magnetically driven accretion reproduce many observational properties of T Tauri stars. For the more massive Herbig Ae/Be stars, the corresponding picture has been questioned lately, in part driven by the fact that their magnetic fields are typically one order of magnitude weaker. Indeed, the search for magnetic fields in Herbig Ae/Be stars has been quite time consuming, with a detection rate of about 10% (e.g. Alecian et al. 2008), also limited by the current potential to detect weak magnetic fields. Over the last two decades, magnetic fields were found in about twenty objects (Hubrig et al. 2015) and for only two Herbig Ae/Be stars was the magnetic field geometry constrained. Ababakr, Oudmaijer & Vink (2017) studied magnetospheric accretion in 56 Herbig Ae/Be stars and found that the behavior of Herbig Ae stars is similar to T Tauri stars, while Herbig Be stars earlier than B7/B8 are clearly different. The origin of the magnetic fields in Herbig Ae/Be stars is still under debate. Potential scenarios include the concentration of the interstellar magnetic field under magnetic flux conservation, pre-main-sequence dynamos during convective phases, mergers, or common envelope developments. The next step in this line of research will be a dedicated observing campaign to monitor about two dozen HAeBes over their rotation cycle.

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The accretion rates and mechanisms of Herbig Ae/Be stars

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa169 ◽

2020 ◽

Vol 493 (1) ◽

pp. 234-249 ◽

Cited By ~ 4

Author(s):

C Wichittanakom ◽

R D Oudmaijer ◽

J R Fairlamb ◽

I Mendigutía ◽

M Vioque ◽

...

Keyword(s):

Accretion Rate ◽

High Mass ◽

Be Stars ◽

Alpha Emission ◽

T Tauri ◽

Accretion Model ◽

Accretion Rates ◽

Theoretical Considerations ◽

Gaia Dr2 ◽

Magnetospheric Accretion

ABSTRACT This work presents a spectroscopic study of 163 Herbig Ae/Be stars. Amongst these, we present new data for 30 objects. Stellar parameters such as temperature, reddening, mass, luminosity, and age are homogeneously determined. Mass accretion rates are determined from $\rm H\alpha$ emission line measurements. Our data is complemented with the X-Shooter sample from previous studies and we update results using Gaia DR2 parallaxes giving a total of 78 objects with homogeneously determined stellar parameters and mass accretion rates. In addition, mass accretion rates of an additional 85 HAeBes are determined. We confirm previous findings that the mass accretion rate increases as a function of stellar mass, and the existence of a different slope for lower and higher mass stars, respectively. The mass where the slope changes is determined tobe $3.98^{+1.37}_{-0.94}\, \rm M_{\odot }$. We discuss this break in the context of different modes of disc accretion for low- and high-mass stars. Because of their similarities with T Tauri stars, we identify the accretion mechanism for the late-type Herbig stars with the Magnetospheric Accretion. The possibilities for the earlier-type stars are still open, we suggest the Boundary Layer accretion model may be a viable alternative. Finally, we investigated themass accretion–age relationship. Even using the superior Gaia based data, it proved hard to select a large enough sub-sample to remove the mass dependence in this relationship. Yet, it would appear that the mass accretion does decline with age as expected from basic theoretical considerations.

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New evidence for weak magnetic fields in Herbig Ae/Be stars

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz2190 ◽

2019 ◽

Cited By ~ 2

Author(s):

S P Järvinen ◽

T A Carroll ◽

S Hubrig ◽

I Ilyin ◽

M Schöller

Keyword(s):

Magnetic Fields ◽

Longitudinal Magnetic Field ◽

Be Stars ◽

Lower Mass ◽

Singular Value Decomposition Method ◽

T Tauri ◽

Weak Magnetic Fields ◽

Be Star ◽

Value Decomposition ◽

New Evidence

Abstract In recent years Herbig Ae/Be stars receive considerable attention as their disks are believed to be the sites of on-going planet formation. Confirming the presence of magnetic fields in these stars is critical for understanding the transport of angular momentum during the protostellar phase. Furthermore, magnetic fields set the conditions for strongly anisotropic accretion. In this study we present the results of our recent observing campaigns of a sample of Herbig Ae/Be stars aimed at measurements of their magnetic fields applying the Singular Value Decomposition method to high resolution spectropolarimetric observations. The strongest longitudinal magnetic field of 209 G is detected in the Herbig Be star HD 58647, whereas the weakest field of 17 G is measured in the Herbig Ae star HD 190073. A change of polarity is detected for HD 58647 and in the Herbig Be star HD 98922. The obtained results provide further evidence that Herbig Ae/Be stars possess much weaker magnetic fields than their lower mass counterpart T Tauri stars with magnetic fields of kG order.

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Variability of the nighttime OH layer and mesospheric ozone at high latitudes during northern winter: influence of meteorology

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-10291-2010 ◽

2010 ◽

Vol 10 (21) ◽

pp. 10291-10303 ◽

Cited By ~ 28

Author(s):

A. Damiani ◽

M. Storini ◽

M. L. Santee ◽

S. Wang

Keyword(s):

Carbon Monoxide ◽

Lower Thermosphere ◽

Complete Disappearance ◽

High Latitudes ◽

Atmospheric Conditions ◽

Short Term ◽

Lower Altitude ◽

Additional Tool ◽

The Third ◽

Short Term Variability

Abstract. Analyses of OH zonal means, recorded at boreal high latitudes by the Aura Microwave Limb Sounder (MLS) in winters of 2005–2009, have shown medium- (weeks) and short- (days) term variability of the nighttime OH layer. Because of the exceptional descent of air from the mesosphere-lower thermosphere (MLT) region, medium-term variability occurred during February 2006 and February/March 2009. The layer normally situated at about 82 km descended by about 5–7 km, and its density increased to more than twice January values. In these periods and location the abundance of the lowered OH layer is comparable to the OH values induced by Solar Energetic Particle (SEP) forcing (e.g., SEP events of January 2005) at the same altitudes. In both years, the descent of the OH layer was coupled with increased mesospheric temperatures, elevated carbon monoxide and an almost complete disappearance of ozone at the altitude of the descended layer (which was not observed in other years). Moreover, under these exceptional atmospheric conditions, the third ozone peak, normally at about 72 km, is shown to descend about 5 km to lower altitude and increase in magnitude, with maximum values recorded during February 2009. Short-term variability occurred during Sudden Stratospheric Warming (SSW) events, in particular in January 2006, February 2008 and January 2009, when dynamics led to a smaller abundance of the OH layer at its typical altitude. During these periods, there was an upward displacement of the OH layer coupled to changes in ozone and carbon monoxide. These perturbations were the strongest during the SSW of January 2009; coincident upper mesospheric temperatures were the lowest recorded over the late winters of 2005–2009. Finally, the series of SSW events that occurred in late January/February 2008 induced noticeable short-term variability in ozone at altitudes of both the ozone minimum and the third ozone peak. These phenomena, confined inside the polar vortex, are an additional tool that can be used to investigate mesospheric vortex dynamics.

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Variability of the nighttime OH layer and mesospheric ozone at high latitudes during northern winter: influence of meteorology

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-10-14583-2010 ◽

2010 ◽

Vol 10 (6) ◽

pp. 14583-14610 ◽

Cited By ~ 3

Author(s):

A. Damiani ◽

M. Storini ◽

M. L. Santee ◽

S. Wang

Keyword(s):

Carbon Monoxide ◽

Lower Thermosphere ◽

Complete Disappearance ◽

High Latitudes ◽

Atmospheric Conditions ◽

Short Term ◽

Lower Altitude ◽

Additional Tool ◽

The Third ◽

Short Term Variability

Abstract. Analyses of OH zonal means, recorded at boreal high latitudes by the Aura Microwave Limb Sounder (MLS), have shown medium- (weeks) and short-term (days) variability of the nighttime OH layer. Because of the exceptional descent of air from the mesosphere-lower thermosphere region, medium-term variability occurred during February 2006 and February/March 2009. The layer normally situated at about 82 km descended by about 5–7 km, and its density increased to more than twice January values. In these periods and location the abundance of the lowered OH layer is comparable with the OH values induced by Solar Energetic Particle (SEP) forcing (e.g. SEP events of January 2005) at the same altitudes. In both years, the drop of the OH layer was coupled with increased mesospheric temperatures, elevated carbon monoxide and an almost complete disappearance of ozone at the altitude of the descended layer (which was not observed in other years). Moreover, under these exceptional atmospheric conditions, the third ozone peak is shown descending to lower altitude and increasing its abundance, with maximum values recorded during February 2009. Short-term variability occurred during Sudden Stratospheric Warming (SSW) events, in particular in January 2006, February 2008 and January 2009, when dynamics led to a smaller abundance of the OH layer at its typical altitude. The upward extension of the OH layer coupled to changes in ozone and carbon monoxide is shown to be strongest during the SSW of January 2009, coincident with the lowest upper mesospheric temperatures recorded at that time of year during 2005–2009. Finally, the series of SSW events that occurred in late January/February 2008 induced noticeable short-term variability in ozone at altitudes of both the ozone minimum and the third ozone peak. These phenomena, confined inside the polar vortex, are an additional tool that can be used to investigate mesospheric vortex dynamics.

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