FY Canis Majoris

FY Canis Majoris (FY CMa), also known as HD 58978, is a star about 1,800 light years from the Earth, in the constellation Canis Major (very near the border of Puppis). It is usually a 5th magnitude star, so it will be visible to the naked eye of an observer far from city lights. It is a Gamma Cassiopeiae variable star, whose brightness varies from magnitude 4.8 to 6.25.

Light curves for FY CMa. The upper panel, showing the long-timescale variability, is plotted from ASAS data,[8] and the lower panels, showing short-timescale variability, is plotted from TESS data.[9]

FY Canis Majoris

Location of FY Canis Majoris (circled)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Canis Major
Right ascension	07h 26m 59.48269s[1]
Declination	−23° 05′ 09.6897″[1]
Apparent magnitude (V)	4.8 - 6.25[2]
Characteristics
Spectral type	B0.5 IVe[3]
Variable type	γ Cas[2]
Astrometry
Proper motion (μ)	RA: −6.692±0.038[1] mas/yr Dec.: +4.054±0.078[1] mas/yr
Parallax (π)	1.7919 ± 0.0645 mas[1]
Distance	1,820 ± 70 ly (560 ± 20 pc)
Absolute magnitude (MV)	−2.23[4]
Orbit[5]
Primary	Be star
Companion	hot subdwarf
Period (P)	37.257±0.003 d
Inclination (i)	>66[6]°
Periastron epoch (T)	2448529.64±0.15
Semi-amplitude (K1) (primary)	14.4±0.9 km/s
Semi-amplitude (K2) (secondary)	128.2±2.2 km/s
Details
Be star
Mass	10-13[6] M☉
Radius	6.8[6] R☉
Luminosity	26,915[6] L☉
Temperature	27,500±3,000[5] K
Rotational velocity (v sin i)	340±40[5] km/s
hot subdwarf
Mass	1.1-1.5[6] M☉
Radius	0.81[6] R☉
Luminosity	2,399[6] L☉
Temperature	45,000±5,000[5] K
Rotational velocity (v sin i)	41±5[5] km/s
Other designations
HD  58978, HIP 36168, HR 2855, SAO 173752, BD−22 1874[7]
Database references
SIMBAD	data

In 1967, Alejandro Feinstein detected low amplitude (0.14 magnitude) variability in HD 58978's brightness, from photoelectric observations at La Plata Observatory.[10] The star was given the variable star designation FY Canis Majoris in 1973.[11] During an outburst in 2009, the star brightened to magnitude 4.8, its historical peak brightness.[2] TESS data show that FY CMa pulsates at many different frequencies, with periods ranging from 1.07 hours to 5.8 days.[12]

In 1905, Edward King noted that FY CMa has a peculiar spectrum with an unusually bright Hβ (and other) emission lines.[13] Paul Merrill et al. listed it as a Be star in 1925.[14] Spectra taken by various observers over several decades of the 20th century showed that the strength and relative intensities of the star's emission line are variable,[3] and could change on a timescale of just a few days.[15]

In 2008, Geraldine Peters et al. obtained high resolution spectra from the IUE and found that the Be star has hot, far less massive subdwarf companion star, with which it forms a spectroscopic binary.[5]

Hui-Lau Cao calculated that FY CMa is losing mass at a rate of about 6 × 10⁻⁸ M_☉/year via a stellar wind with a terminal velocity of about 750 km/sec.[16]