RR Caeli

RR Caeli is a double star in the constellation Caelum. It is approximately 66 light years from Earth.[4] It was first noted to be a high-proper motion star in 1955 by Jacob Luyten, and given the name LFT 349. Discovered to be an eclipsing binary in 1979, it has a baseline magnitude of 14.36, dimming markedly every 7.2 hours for an interval of around 10 minutes, due to the total eclipse of the brighter star by the fainter one. Its variability in brightness led to its being given the variable star designation RR Caeli in 1984.[5] This star system consists of a red dwarf of spectral type M6 and a white dwarf that orbit each other every seven hours; the former is 18% as massive as the Sun, while the latter has 44% of the Sun's mass.[5] The red dwarf is tidally locked with the white dwarf, meaning it displays the same side to the heavier star.[6] The system is also a post-common-envelope binary, and the red dwarf star is transferring material onto the white dwarf. In approximately 9–20 billion years, RR Caeli will likely become a cataclysmic variable star due to the period's gradual shortening, leading to increasing rates of transfer of hydrogen to the surface of the white dwarf.[3] The white dwarf is likely to have a plain helium core, as its density is too low for the carbon-oxygen core.[1]

RR Caeli

Observation data Epoch J2000      Equinox J2000
Constellation	Caelum
Right ascension	04^h 21^m 05.563^s
Declination	−48° 39′ 07.02″
Apparent magnitude (V)	14.40
Characteristics
White dwarf
Evolutionary stage	White dwarf
Spectral type	DA7.8
U−B color index	−0.42
B−V color index	0.52
Red dwarf
Evolutionary stage	main-sequence star
Spectral type	M4[1]
Astrometry
Proper motion (μ)	RA: 30.030 mas/yr Dec.: −533.886 mas/yr
Parallax (π)	47.2000 ± 0.0200[2] mas
Distance	69.10 ± 0.03 ly (21.186 ± 0.009 pc)
Orbit[1]
Primary	White dwarf
Companion	Red dwarf
Period (P)	7.289 h
Details[3]
White dwarf
Mass	0.448±0.002[1] M☉
Radius	0.01568±0.00009[1] R☉
Luminosity (bolometric)	0.0007 ± 0.00015[note 1] L☉
Surface gravity (log g)	(7.67–7.72) ± 0.06 cgs
Temperature	7540 ± 175 K
Red dwarf
Mass	(0.182–0.183) ± 0.012 M☉
Radius	(0.203–0.215) ± 0.015 R☉
Luminosity (bolometric)	0.0038 ± 0.0013[note 1] L☉
Surface gravity (log g)	(5.04–5.09) ± 0.04 cgs
Temperature	3100 ± 113 K
Other designations
RR Caeli, LFT 349, WD 0419-487, GJ 2034 AB, 2MASS J04210556-4839070, GSC 08072-01454, Gaia EDR3 4788741548375134336
Database references
SIMBAD	data
Exoplanet Archive	data
ARICNS	data
Extrasolar Planets Encyclopaedia	data

Planetary system

In 2012, analysis of slight variations in the observed light curve of the system showed that there was almost certainly a giant planet about four times as massive as Jupiter orbiting the pair of stars with a period of 11.9 years, and that there was evidence for a second possible substellar body further out as well.[7] More observations of the light curve are likely to help confirm the presence of one or both planets.[7]

The RR Caeli planetary system[7]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (years)	Eccentricity	Inclination	Radius
b	≥4.2 ± 0.4 MJ	~5.3 ± 0.6	11.9 ± 0.1	0	>17.6°	—

Notes

1. From ${\displaystyle {\begin{smallmatrix}L=4\pi R^{2}\sigma T_{\rm {eff}}^{4}\end{smallmatrix}}}$, where ${\displaystyle {\begin{smallmatrix}L\end{smallmatrix}}}$ is the luminosity, ${\displaystyle {\begin{smallmatrix}R\end{smallmatrix}}}$ is the radius, ${\displaystyle {\begin{smallmatrix}T_{\rm {eff}}\end{smallmatrix}}}$ is the effective surface temperature and ${\displaystyle {\begin{smallmatrix}\sigma \end{smallmatrix}}}$ is the Stefan–Boltzmann constant.

References

1. Testing the white dwarf mass-radius relationship with eclipsing binaries, 2017, arXiv:1706.05016
2. Brown, A. G. A.; et al. (Gaia collaboration). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics (in press). arXiv:2012.01533. doi:10.1051/0004-6361/202039657. S2CID 227254300. Gaia EDR3 record for this source at VizieR.
3. Maxted, P. F. L.; O'Donoghue, D.; Morales-Rueda, L.; Napiwotzki, R.; Smalley, B. (2007). "The mass and radius of the M-dwarf in the short-period eclipsing binary RR Caeli". Monthly Notices of the Royal Astronomical Society. 376 (2): 919–928. arXiv:astro-ph/0702005. Bibcode:2007MNRAS.376..919M. doi:10.1111/j.1365-2966.2007.11564.x. S2CID 3569936.
4. "V* RR Caeli – Eclipsing binary of Algol type (detached)". SIMBAD. Retrieved 18 February 2013.
5. Bruch, A.; Diaz, M. P. (1998). "The Eclipsing Precataclysmic Binary RR Caeli". The Astronomical Journal. 116 (2): 908–916. Bibcode:1998AJ....116..908B. doi:10.1086/300471.
6. Ribeiro, T.; Baptista, R.; Kafka, S.; Dufour, P.; Gianninas, A.; Fontaine, G. (2013). "Accretion and activity on the post-common-envelope binary RR Caeli". Astronomy & Astrophysics. 556A: A34. arXiv:1307.5921. Bibcode:2013A&A...556A..34R. doi:10.1051/0004-6361/201220340. S2CID 55229588.
7. Qian, S. B.; Liu, L.; Zhu, L. Y.; Dai, Z. B.; Fernández Lajús, E.; Baume, G. L. (2012). "A circumbinary planet in orbit around the short-period white dwarf eclipsing binary RR Cae". Monthly Notices of the Royal Astronomical Society: Letters. 422 (1): L24–L27. arXiv:1201.4205. Bibcode:2012MNRAS.422L..24Q. doi:10.1111/j.1745-3933.2012.01228.x. S2CID 119190656.