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HD 93521
	; A light curve for HD 93521, plotted from TESS data
Observation data; Epoch J2000.0 Equinox J2000.0
Constellation	Leo Minor
Right ascension	10h 48m 23.511s
Declination	37° 34′ 13.08″
Apparent magnitude (V)	7.03
Characteristics
Spectral type	O9.5IIInn or O9.5Vp
B−V color index	−0.227±0.007
Astrometry
Radial velocity (Rv)	−14.1±3.2 km/s
Proper motion (μ)	RA: −0.162 mas/yr ; Dec.: −1.398 mas/yr
Parallax (π)	0.6577 ± 0.0333 mas
Distance	5,000 ± 300 ly ; (1,520 ± 80 pc)
Details
Mass	17 M☉
Radius	Polar: 6.1±0.3 R☉; Equator: 7.4±0.4 R☉
Luminosity	39,800+10,300; −8,200 L☉
Surface gravity (log g)	Polar: 4.1±0.4 cgs; Equator: 3.7±0.4 cgs
Temperature	Polar: 34,600±1,200 K; Equator: 28,700±1,200 K
Rotational velocity (v sin i)	435±20 km/s
Age	<34 Myr
Other designations
	BD+38°2179, GC 14866, HD 93521, HIP 52849, SAO 62257, PPM 75477
Database references
SIMBAD	data

HD 93521 is a single,^[9] massive star in the northern constellation of Leo Minor. With an apparent visual magnitude of 7.03,^[2] it is too faint to be seen with the naked eye. The star is located at a distance of approximately 5.0 kilolight-years (1.52 kpc) from the Sun based on parallax measurements.^[3] It is positioned at a high galactic latitude of +62°^[10] and is located about 4.6 kilolight-years (1.4 kpc) above the Galactic plane.^[10]

The spectrum of this star matches an O-type main-sequence star with a stellar classification of O9.5V.^[10] It is unusual for a star of this class to have formed so far away from the galaxy's star forming regions.^[9] Absorption lines in its spectrum indicate a metallicity that is inconsistent with being a population II star,^[10] which are typically found in the galaxy halo. Likewise, HD 93521 is unlikely to be a runaway star or a hot subdwarf, either of which could explain its remote location. It may instead be a blue straggler that was formed as a result of a merger.^[9] The resulting star likely began as a close binary system of lower mass, longer-lived stars that were ejected from the galactic disk. The merger would then have reset the evolutionary clock, producing a hotter, shorter-lived star.^[7]

HD 93521 is one of the most rapidly rotating stars known,^[11] with estimates of its projected rotational velocity ranging from 390^[10] up to 435 km/s.^[7] This is at least 90% of the star's breakup velocity, assuming it is being viewed from along the equator.^[10] The rapid spin is creating an equatorial bulge with the radius at the equator being an estimated 7.4 times the Sun's radius while the polar radius is 6.1 times that of the Sun. Due to gravity darkening, the surface temperature at the poles is 34,600 K while the temperature at the equator is only 28,700 K.^[7]

The star may be undergoing mass loss from its stellar wind^[12] and have an equatorial disk of orbiting gas. The star shows evidence of non-radial pulsations, which may be the result of a more rapidly rotating core.^[10]

The brightness, position, and rapid rotation of this star makes it particularly suited to examining the interstellar gas in the Milky Way halo.^[13]

References

^ MAST: Barbara A. Mikulski Archive for Space Telescopes, Space Telescope Science Institute, retrieved 19 September 2022.
^ ^a ^b ^c ^d Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644.
^ ^a ^b ^c ^d ^e Brown, A. G. A.; et al. (Gaia collaboration) (2021), "Gaia Early Data Release 3: Summary of the contents and survey properties", Astronomy & Astrophysics, 649: A1, arXiv:2012.01533, Bibcode:2021A&A...649A...1G, doi:10.1051/0004-6361/202039657, S2CID 227254300 (Erratum: doi:10.1051/0004-6361/202039657e). Gaia EDR3 record for this source at VizieR.
^ Sota, A.; et al. (April 2011), "The Galactic O-Star Spectroscopic Survey. I. Classification System and Bright Northern Stars in the Blue-violet at R ~ 2500", The Astrophysical Journal Supplement, 193 (2): 50, arXiv:1101.4002, Bibcode:2014MNRAS.441..910R, doi:10.1088/0067-0049/193/2/24, S2CID 119248206, 24.
^ Münch, Guido; Zirin, Harold (January 1961), "Interstellar Matter at Large Distances from the Galactic Plane", Astrophysical Journal, 133: 11, Bibcode:1961ApJ...133...11M, doi:10.1086/146999.
^ Gontcharov, G. A. (November 2006), "Pulkovo Compilation of Radial Velocities for 35495 Hipparcos stars in a common system", Astronomy Letters, 32 (11): 759–771, arXiv:1606.08053, Bibcode:2006AstL...32..759G, doi:10.1134/S1063773706110065, S2CID 119231169.
^ ^a ^b ^c ^d ^e Gies, Douglas R.; et al. (February 2022), "The Transformative Journey of HD 93521", The Astronomical Journal, 163 (2): 7, arXiv:2112.08235, Bibcode:2022AJ....163..100G, doi:10.3847/1538-3881/ac43be, S2CID 245144736, 100.
^ "HD 93521", SIMBAD, Centre de données astronomiques de Strasbourg, retrieved 2022-05-07.
^ ^a ^b ^c Rauw, G.; Morel, T.; Palate, M. (October 2012), "The nature of the high Galactic latitude O-star HD 93521: new results from X-ray and optical spectroscopy", Astronomy & Astrophysics, 546: 12, arXiv:1209.2606, Bibcode:2012A&A...546A..77R, doi:10.1051/0004-6361/201219865, S2CID 54618518, A77.
^ ^a ^b ^c ^d ^e ^f ^g Rauw, G.; et al. (August 2008), "Spectroscopic and photometric variability of the O9.5 Vp star HD 93521", Astronomy and Astrophysics, 487 (2): 659–670, arXiv:0806.3640, Bibcode:2008A&A...487..659R, doi:10.1051/0004-6361:200810002, S2CID 18164022.
^ Ramiaramanantsoa, Tahina; et al. (June 2014), "MOST detects corotating bright spots on the mid-O-type giant ξ Persei", Monthly Notices of the Royal Astronomical Society, 441 (1): 910–917, arXiv:1403.7843, Bibcode:2014MNRAS.441..910R, doi:10.1093/mnras/stu619.
^ Bjorkman, J. E.; Ignace, R.; Tripp, T. M.; Cassinelli, J. P. (May 1994), "Evidence for a Disk in the Wind of {HD93521}: UV Line Profiles from an Axisymmetric Model", Bulletin of the American Astronomical Society, 26, American Astronomical Society, 184th AAS Meeting, id.31.07: 909, Bibcode:1994AAS...184.3107B.
^ Barnstedt, J.; Gringel, W.; Kappelmann, N.; Grewing, M. (April 2000), "The ORFEUS II Echelle spectrum of HD 93521: A reference for interstellar molecular hydrogen", Astronomy and Astrophysics Supplement, 143 (2): 193–210, arXiv:astro-ph/0006294, Bibcode:2000A&AS..143..193B, doi:10.1051/aas:2000176, S2CID 17500722.

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Rzaev, A. Kh. (December 2007), "Some features of the radial-velocity variations of lines of different intensity in the spectrum of HD 93521. Variability of the stellar wind", Astrophysical Bulletin, 62 (4): 331–338, Bibcode:2007AstBu..62..331R, doi:10.1134/S1990341307040049, S2CID 119547333.
Danezis, A.; Antoniou, E.; Lyratzi, E.; Nikolaidis, D.; Popović, L. C.; Dimitrijević, M. S.; Theodossiou, E. (July 2007), Gómez de Castro, Ana I.; Barstow, Martin A. (eds.), "Long term variability of the coronal and post - coronal C IV region of the Oe star HD 93521" (PDF), UV Astronomy: Stars from Birth to Death. Proceedings of the Joint Discussion n.4 during the IAU general Assembly of 2006, p. 113, Bibcode:2007uasb.conf..113D, ISBN 978-84-7491-852-6, retrieved 2022-05-08.^{[dead link‍]}
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Hausen, N. R.; Reynolds, R. J.; Haffner, L. M.; Tufte, S. L. (February 2002), "Interstellar Hα Line Profiles toward HD 93521 and the Lockman Window", The Astrophysical Journal, 565 (2): 1060–1068, arXiv:astro-ph/0112500, Bibcode:2002ApJ...565.1060H, doi:10.1086/324692, S2CID 6990680.
Gringel, W.; Barnstedt, J.; de Boer, K. S.; Grewing, M.; Kappelmann, N.; Richter, P. (June 2000), "ORFEUS II Echelle spectra: molecular hydrogen at high velocities toward HD 93521", Astronomy and Astrophysics, 358: L37 – L40, arXiv:astro-ph/0006189, Bibcode:2000A&A...358L..37G.
Howarth, Ian D.; Townsend, R. H. D.; Clayton, M. J.; Fullerton, A. W.; Gies, D. R.; Massa, D.; Prinja, R. K.; Reid, A. H. N. (June 1998), "Time-dependent structure in ultraviolet absorption lines of the rapid rotators HD 64760 (B0Ib) and HD 93521 (O9.5V)", Monthly Notices of the Royal Astronomical Society, 296 (4): 949–960, Bibcode:1998MNRAS.296..949H, doi:10.1046/j.1365-8711.1998.01437.x.
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Irvine, Nelson J. (February 1989), "HD 93521: A High-Latitude Shell Star", Astrophysical Journal Letters, 337: L33, Bibcode:1989ApJ...337L..33I, doi:10.1086/185372.
Ramella, M.; Morossi, C.; Santin, P. (October 1980), "Ultraviolet observations of the blue halo star : HD 93521.", Astronomy and Astrophysics, 90: 146–150, Bibcode:1980A&A....90..146R.
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