ER Ursae Majoris

ER Ursae Majoris is a variable star in the northern circumpolar constellation of Ursa Major, abbreviated ER UMa. It is a prototype system for a subclass of SU Ursae Majoris dwarf novae.^[5] The system ranges in brightness from a peak apparent visual magnitude of 12.4 down to 15.2,^[3] which is too faint to be visible to the naked eye. The distance to this system, based on parallax measurements, is approximately 1,163 light years.^[2]

ER Ursae Majoris

A visual band light curve for ER Ursae Majoris. The main plot shows a normal outburst in 2004, with the estimated value for the unobserved maximum plotted in red. The inset plot shows superhump oscillations. Adapted from Zhao et al. (2006).[1]
Observation data Epoch J2000      Equinox J2000
Constellation	Ursa Major
Right ascension	09h 47m 11.941s[2]
Declination	+51° 54′ 08.95″[2]
Apparent magnitude (V)	12.4 to 15.2[3]
Characteristics
Variable type	Dwarf nova(?)[3]
Astrometry
Proper motion (μ)	RA: 33.659 mas/yr[2] Dec.: −6.209 mas/yr[2]
Parallax (π)	2.8039±0.0205 mas[2]
Distance	1,163 ± 9 ly (357 ± 3 pc)
Orbit[4]
Period (P)	0.06366±0.00003 d
Eccentricity (e)	0.00 (assumed)
Inclination (i)	18–50[5]°
Periastron epoch (T)	2,449,740.0637±0.0008 HJD
Argument of periastron (ω) (secondary)	0.00 (assumed)°
Semi-amplitude (K1) (primary)	48±4 km/s
Details
White dwarf
Mass	1.0±0.2[5] M☉
Temperature	32,000[5] K
Donor star
Mass	0.10[5] M☉
Other designations
PG 0943+521, ER UMa, AAVSO 0939+52, GSC 03439-00550, 2MASS J09471193+5154089[6]
Database references
SIMBAD	data

This system was identified as an ultraviolet excess object as part of the Palomar-Green (PG) survey by R. F. Green and associate in 1986. It was given the catalog identifier PG 0943+521, and was confirmed to be cataclysmic variable. In 1992, it was determined this is a dwarf nova that ranges in brightness from magnitude 12.3 down to 15.2. F. A. Ringwald in 1993 found a candidate orbital period of 0.1997 days based on radial velocity variation, but with some uncertainty.^[7]

In 1995, T. Kato and C. Kunjaya confirmed this is a SU Ursae Majoris-type dwarf nova, and noted the unusual nature of this system, finding it has a long superoutburst lasting about 20 days and the supercycle (the time between superoutbursts) is very short at around 43 days.^[8] Large amplitude superhumps were found to occur near the start of a superoutburst, with a brightness increase of around 0.35 magnitude.^[9] The properties of the system suggest a high mass transfer rate and the white dwarf component is hotter than in other typical dwarf novae. During periods of quiescence, the accretion rate is 7.3×10⁻¹¹ M_☉·yr⁻¹.^[5]