BH Crucis

BH Crucis, also known as Welch's Red Variable, is a star in the constellation Crux. A long period (Mira-type) variable, its apparent magnitude ranges from 6.6 to 9.8 over 530 days.[2] Hence at its brightest it is barely visible with the unaided eye in a rural sky.[6] A red giant, it had been classified by SIMBAD as ranging between spectral types SC4.5/8-e and SC7/8-e,[2] but appears to have evolved into a C-type (carbon star) spectrum by 2011.[4]

BH Crucis
Observation data
Epoch 2000.0      Equinox 2000.0
Constellation Crux
Right ascension  12h 16m 16.7898187344s[1]
Declination −56° 17′ 09.627987388″[1]
Apparent magnitude (V) 6.55 - 10.1[2]
Characteristics
Evolutionary stage Red giant
Spectral type SC4.5/8-e - SC7/8-e[2]
Variable type Mira[2]
Astrometry
Proper motion (μ) RA: -5.009 ± 0.065[1] mas/yr
Dec.: -4.141 ± 0.053[1] mas/yr
Parallax (π)0.9952 ± 0.0529[1] mas
Distance3,300 ± 200 ly
(1,000 ± 50 pc)
Absolute magnitude (MV)−4.80[3]
Details
Radius216[1] R☉
Luminosity13,700[4] L☉
Temperature3,000[4] K
Other designations
Welch's Red Variable, BH Cru, HIP 59844[5]
Database references
SIMBADdata

Ronald G. Welch discovered the star while looking for new variables in October 1969. In the first thirty years since discovery, it has become redder and brighter (mean magnitude changing from 8.047 to 7.762) and its period lengthened by 25% from 421 to 530 days.[7] Retrospective examination of photographic plates taken in South West Africa in 1937 and 1951 and stored at Sonneberg Observatory suggest the amplitude of variation might have been smaller in the earliest records.[8] A study of the star published in 2011 found that the increase in period appeared to have stopped or even begun reversing (estimated at 524 days in 2011), and that the spectral class had changed from SC to C, with carbon emission becoming more prominent. Technetium was also recorded in the emission spectrum and its surface temperature deemed to have cooled to 3000 K.[4] Unusually for a Mira variable, BH Crucis had a double maximum, with two peaks in brightness, reminiscent of an RV Tauri variable. However, with the lengthening of its period, this feature disappeared.[7]

Guandalini and Cristallo calculated the luminosity of Mira variables based on their periods. Using a period of 421 days, they calculated the absolute magnitude of BH Crucis to be -4.80.[3] Uttenthaler and colleagues calculated a bolometric magnitude of -5.59.[4] Gaia Data Release 2 gives a parallax of 0.9952 mas and a corresponding distance of around 1000 pc.[1]

References
  1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  2. Otero, Sebastian (6 January 2011). "BH Crucis". AAVSO Website. American Association of Variable Star Observers. Retrieved 23 June 2014.
  3. Guandalini, R.; Cristallo, S. (2013). "Luminosities of carbon-rich asymptotic giant branch stars in the Milky Way". Astronomy & Astrophysics. 555: 7. arXiv:1305.4203. Bibcode:2013A&A...555A.120G. doi:10.1051/0004-6361/201321225. A120.
  4. Uttenthaler, S.; van Stiphout, K.; Voet, K.; van Winckel, H.; van Eck, S.; Jorissen, A.; Kerschbaum, F.; Raskin, G.; Prins, S.; Pessemier, W.; Waelkens, C.; Frémat, Y.; Hensberge, H.; Dumortier, L.; Lehmann, H. (2011). "The evolutionary state of Miras with changing pulsation periods". Astronomy & Astrophysics. 531: 11. arXiv:1105.2198. Bibcode:2011A&A...531A..88U. doi:10.1051/0004-6361/201116463. A88.
  5. "V* BH Cru". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 7 December 2018.
  6. Bortle, John E. (February 2001). "The Bortle Dark-Sky Scale". Sky & Telescope. Sky Publishing Corporation. Retrieved 25 June 2014.
  7. Walker, W.S.G. (2009). "BH Crucis : Period, Magnitude, and Color Changes". J. Am. Assoc. Variable star obs. 37: 87–95. Bibcode:2009JAVSO..37...87W.
  8. Walker, W.S.G.; Marino, Brian F. (1972). "Welch's red variable star in Crux an R Centauri like variable". IAU Inform. Bull. Var. Stars. 679: 1. Bibcode:1972IBVS..679....1W.
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