X Cancri

Not to be confused with χ (Chi) Cancri.

X Cancri is a variable star in the northern constellation of Cancer.^[8] It has a red hue and is visible to the naked eye at its brightest. The distance to this object is approximately 1,860 light years based on parallax measurements, but is drifting closer with a radial velocity of −5 km/s. It lies very close to the ecliptic and so is subject to lunar occultations.^[9]

A visual band light curve for X Cancri, adapted from Percy et al. (2001)^[10]

X Cancri

Location of X Cancri (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Cancer
Right ascension	08h 55m 22.88194s[1]
Declination	+17° 13′ 52.5830″[1]
Apparent magnitude (V)	5.52 – 7.50[2]
Characteristics
Spectral type	C-N4.5 C25.5 MS3[3]
B−V color index	+2.975±0.039[4]
Variable type	SRb[2]
Astrometry
Radial velocity (Rv)	−5.0±0.7[4] km/s
Proper motion (μ)	RA: −2.076[1] mas/yr Dec.: +6.457[1] mas/yr
Parallax (π)	1.7500 ± 0.0356 mas[1]
Distance	1,860 ± 40 ly (570 ± 10 pc)
Absolute magnitude (MV)	−4.96[5]
Details
Radius	208[6] R☉
Luminosity	4,646[6] L☉
Surface gravity (log g)	−0.014[6] cgs
Temperature	3,239[6] K
Metallicity [Fe/H]	−0.3[7] dex
Other designations
X Cnc, BD+17°1973, GC 12322, HD 76221, HIP 43811, HR 3541, SAO 98230[8]
Database references
SIMBAD	data

This object is a carbon star – an aging red giant star on the asymptotic giant branch that has a higher abundance of carbon than oxygen in its atmosphere^[5] – and is one of the brightest carbon stars in the sky.^[11] It has a carbon star spectral classification of C-N4.5 C₂5.5 MS3.^[3] The first C indicates that it is a carbon star, and the N5 that it is a fairly cool strongly red AGB star. The C₂ index indicates the strength of the Swan bands on a scale of one to eight, which shows the relative abundance of carbon vs oxygen. The MS index, not to be confused with an MS spectral class, indicates the strength of the SiC₂ bands on a scale of one to seven. These bands are thought to be very sensitive to temperature.^[12]

This is a semiregular variable star of subtype SRb that ranges in brightness from visual magnitude 5.52 down to 7.50 with a period of 193 days.^[2] Fourier analysis has shown that the star also pulsates with periods of 350 and 1,870 days.^[13]

The angular diameter of X Cancri has been measured using both lunar occultations and very long baseline interferometry, both methods giving a diameter around 8 mas.^[14]

References

1. Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
2. Samus, N. N.; et al. (2017), "General Catalogue of Variable Stars", Astronomy Reports, 5.1, 61 (1): 80–88, Bibcode:2017ARep...61...80S, doi:10.1134/S1063772917010085, S2CID 125853869.
3. Barnbaum, Cecilia; et al. (1996), "A Moderate-Resolution Spectral Atlas of Carbon Stars: R, J, N, CH, and Barium Stars", Astrophysical Journal Supplement, 105: 419, Bibcode:1996ApJS..105..419B, doi:10.1086/192323.
4. 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.
5. 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, S2CID 54918450, A120.
6. McDonald, I.; Zijlstra, A. A.; Watson, R. A. (2017), "Fundamental parameters and infrared excesses of Tycho-Gaia stars", Monthly Notices of the Royal Astronomical Society, 471 (1): 770, arXiv:1706.02208, Bibcode:2017MNRAS.471..770M, doi:10.1093/mnras/stx1433, S2CID 73594365.
7. Lambert, David L.; et al. (October 1986), "The Chemical Composition of Carbon Stars. I. Carbon, Nitrogen, and Oxygen in 30 Cool Carbon Stars in the Galactic Disk", Astrophysical Journal Supplement, 62: 373, Bibcode:1986ApJS...62..373L, doi:10.1086/191145.
8. "X Cnc", SIMBAD, Centre de données astronomiques de Strasbourg, retrieved 2020-02-26.
9. Schmidtke, P. C.; Africano, J. L. (January 2011), "KPNO Lunar Occultation Summary. III", The Astronomical Journal, 141 (1): 7, Bibcode:2011AJ....141...10S, doi:10.1088/0004-6256/141/1/10, 10.
10. Percy, John R.; Wilson, Joseph B.; Henry, Gregory W. (August 2001), "Long-Term VRI Photometry of Small-Amplitude Red Variables. I. Light Curves and Periods", Publications of the Astronomical Society of the Pacific, 113 (786): 983–996, Bibcode:2001PASP..113..983P, doi:10.1086/322153, S2CID 14609175.
11. Upgren, A. R. (February 1973), "The Parallax and Proper Motion of the Carbon Star X CANCRI", Astrophysical Journal, 179: L121, Bibcode:1973ApJ...179L.121U, doi:10.1086/181130.
12. Keenan, Philip C. (1993), "Revised MK Spectral Classification of the Red Carbon Stars", Publications of the Astronomical Society of the Pacific, 105: 905, Bibcode:1993PASP..105..905K, doi:10.1086/133252.
13. Kiss, L. L.; Szatmáry, K.; Cadmus, R. R.; Mattei, J. A. (1999), "Multiperiodicity in semiregular variables. I. General properties", Astronomy and Astrophysics, 346: 542, arXiv:astro-ph/9904128, Bibcode:1999A&A...346..542K.
14. Richichi, A.; Percheron, I. (2002), "CHARM: A Catalog of High Angular Resolution Measurements", Astronomy and Astrophysics, 386 (2): 492–503, Bibcode:2002A&A...386..492R, doi:10.1051/0004-6361:20020236, hdl:1887/7492.