Be-star rotation: how close to critical?

By R. Townsend, S. Owocki, I. Howarth. 2003

270 citations recorded by the source corpus.

We argue that, in general, observational studies of Be-star rotation have paid insufficient attention to the effects of equatorial gravity darkening. We present new line-profile calculations that emphasize the insensitivity of line width to rotation for fast rotators. Coupled with a critical review of observational procedures, these calculations suggest that the observational parameter v sin i may systematically underestimate the true projected equatorial rotation velocity, v e sin i, by some tens of per cent for rapid rotators. A crucial implication of this work is that Be stars may be rotating much closer to their critical velocities than is generally supposed, bringing a range of new processes into contention for the elusive physical mechanism responsible for the circumstellar disc thought to be central to the Be phenomenon.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/a47e90b35c8fcc9d1dfdde55c4a386e70559863e

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Citations in the corpus

The IACOB project - I. Rotational velocities in northern Galactic O- and early B-type stars revisited. The impact of other sources of line-broadening (2013, background): “– Equatorial gravity darkening will produce an obscuration of the profile wings associated with the larger velocities and thus an underestimation of the actualv sini (Townsend et al., 2004; Frémat et al., 2005).”
A STELLAR ROTATION CENSUS OF B STARS: FROM ZAMS TO TAMS (2010, background): “However, Townsend et al. (2004) point out that very rapidly rotating stars may experience strong gravitational darkening towards their equatorial zones that leads to an underestimate of the actual V sin i value.”
The Progenitor Stars of Gamma-Ray Bursts (2005, background): “Indeed Be stars are the most rapidly rotating of all non-compact stars (Townsend et al. 2004).”
Be/X-ray binaries (2011, background): “As a result, the observational parameter v sin i may systematically underestimate the true projected equatorial rotation velocity (Townsend et al. 2004).”
The VLT-FLAMES survey of massive stars: constraints on stellar evolution from the chemical compositions of rapidly rotating Galactic and Magellanic Cloud B-type stars (2009, methodology): “For this ratio we estimate the change in B magnitude between pole-on (i = 0◦) and equator-on (i = 90◦) views to be only 0.36mag, using the methodology outlined by Townsend et al. (2004).”
Effects of gravitational darkening on the determination of fundamental parameters in fast-rotating B-type stars (2005, methodology): “(3) in Townsend et al. 2004).”
Fast rotating massive stars and the origin of the abundance patterns in galactic globular clusters (2006, background): “The origin of the disk is probably stellar rotation near the critical limit as convincingly discussed by Townsend et al. (2004).”
Formation of Double Neutron Star Systems (2017, background): “If the system does not break up due to the SN (Section 6), its observational counterparts include the so-called Be-star X-ray binaries (Be-HMXBs, Section 3.2), where the Be-nature of the core-hydrogen burning component signals its state of near-critical rotation (Townsend et al. 2004).”
THE ROTATION RATES OF MASSIVE STARS: THE ROLE OF BINARY INTERACTION THROUGH TIDES, MASS TRANSFER, AND MERGERS (2012, methodology): “Although this treatment is very simple, it reproduces the detailed simulations of Townsend et al. (2004) accurately enough for our purpose.”
Rotating massive main-sequence stars: II. Simulating a population of LMC early B-type stars as a test of rotational mixing (2011, background): “Martayan et al. (2008) finds Ω/Ωc = 85% in the LMC, and Townsend et al. (2004) argue that Be-stars might actually rotate as fast as 95% of the breakup velocity, since the line broadening due to rotation becomes insensitive to gravity darkening at high rotational velocities.”
Classical Be stars (2013, background): “2.3, affects the determination of the rotational velocity v sin i, and hence W , because the most rapidly rotating part of the star is becoming inconspicuous as W 0.75 (see, e.g., Stoeckley 1968; Townsend et al. 2004, as well as the left panel of Fig.”
Evolution towards the critical limit and the origin of Be stars (2007, background): “If the star is rotating withυeq/υcrit ∼ 0.95, the additional velocity necessary to launch material into orbit is of the same order as the speed of sound in the outer layers (Townsend et al. 2004).”
A CHARA ARRAY SURVEY OF CIRCUMSTELLAR DISKS AROUND NEARBY Be-TYPE STARS (2013, background): “…are generally fast rotators, but it is difficult to determine the actual equatorial rotational velocity Vrot from the observed projected rotational velocity V sin i (measured from the rotational Doppler broadening of the lines; Townsend et al. 2004) because of the unknown stellar inclination i.”
The Evolutionary Status of Be Stars: Results from a Photometric Study of Southern Open Clusters (2005, methodology): “This rapid rotation may be combined with weaker processes, such as non-radial pulsations or magnetic fields (Porter & Rivinius 2003), to move material from the stellar surface into a disk (Townsend et al. 2004).”