HD 190655

HD 190655

Location of HD 190655 (circled)
Observation data Epoch J2000.0[1]      Equinox J2000.0[1]
Constellation	Cygnus
Right ascension	20^h 04^m 11.34545^s
Declination	+45° 05′ 15.3640″
Apparent magnitude (V)	10.114 (combined)[2]
Characteristics
Evolutionary stage	Red-giant branch star
Spectral type	K2III + (G4IV + M8-L3)[3]
B−V color index	1.153[2]
J−H color index	0.521[4]
J−K color index	0.684[4]
Variable type	Eclipsing binary
Astrometry
Radial velocity (Rv)	−24.1724±0.002728[5] km/s
Proper motion (μ)	RA: -8.419[1] mas/yr Dec.: −3.176[1] mas/yr
Parallax (π)	2.1394 ± 0.3070 mas[1]
Distance	approx. 1,500 ly (approx. 470 pc)
Orbit[3]
Primary	HD 190655 A
Companion	HD 190655 B
Semi-major axis (a)	0.4" (270 AU)
Orbit[3]
Primary	HD 190655 B
Companion	HD 190655 C
Period (P)	5.566513±0.000043 d
Semi-major axis (a)	0.0720±0.0013 AU
Inclination (i)	82.50+0.58 −0.53°
Details[3]
HD 190655 A
Mass	1.69+0.14 −0.10 M☉
Radius	11.13+0.32 −0.26 R☉
Luminosity	43.3±9.5[6] L☉
Surface gravity (log g)	2.577+0.011 −0.009 cgs
Temperature	4720±120 K
Metallicity [Fe/H]	0.143±0.065 dex
Age	2.05+0.56 −0.28 Gyr
HD 190655 B
Mass	1.608+0.087 −0.088 M☉
Radius	3.61+0.11 −0.12 R☉
Luminosity	11.5±1.0 L☉
Temperature	5600±100 K
HD 190655 C
Mass	66.1+4.1 −3.2 MJup
Radius	1.524+0.070 −0.072 RJup
Other designations
BD+44° 3330, Gaia DR3 2082133182277361152, HD 190655, WDS J20042+4505AB, KOI-3886, KIC 8848288, TIC 185060864, TYC 3559-2080-1, GSC 03559-02080, 2MASS J20041135+4505154, WISE J200411.34+450515.3[4]
Database references
SIMBAD	data

HD 190655 (KOI-3886) is a hierarchical triple star system located about 1,500 light-years (460 parsecs) away in the northern constellation of Cygnus. It has a combined apparent magnitude of 10.114, making it readily visible using a small telescope with an aperture of 35 mm or larger,[7] but too faint to observe via the naked eye or binoculars.[8]

The system consists of a K-type red giant (HD 190655 A) and a G-type subgiant in a wide binary, with a transiting brown dwarf orbiting the latter. The brown dwarf component is notable in that it is one of the most irradiated objects of its kind, and also the first to be discovered transiting an evolved star. Since it showed planet-like characteristics in a variety of past observations, the brown dwarf has been dubbed the "ultimate planet impostor."[3]

Stellar components

The two stellar components of the system have both evolved past the main sequence, with similar masses of 1.6–1.7 M_☉. Because of their similar proper motions and an exceedingly low (0.04%) probability of two unrelated evolved stars being situated at such a small separation, the two stars are almost certainly gravitationally bound, i.e., are in a physical binary system.[3]

The primary star, HD 190655 A, is in the process of ascending the red-giant branch, having expanded to 11 times the girth of the Sun. At an effective temperature of 4,720 K (4,450 °C; 8,040 °F), it radiates close to 45 times the luminosity of the Sun from its photosphere.[6] It is aged about two billion years,[3] less than half the age of the Solar System, but has evolved beyond the main sequence unlike the Sun due to its higher mass. Much like most red giants, the star exhibits solar-like oscillations, strongest at a frequency of about 50 μHz.[6]

The secondary star, situated 270 AU away from the primary, is slightly lighter and less evolved, still at the subgiant stage. It is smaller and hotter than the brighter red giant, with a temperature of 5,600 K (5,330 °C; 9,620 °F), similar to that of the Sun (5,772 K[9]). With a diameter of 3.61 R_☉, it shines at a luminosity of 11.5 L_☉, about a quarter of the primary star.[3]

In 2013, the pulsating primary star was examined for the possibility of being part of an eclipsing binary, but was deemed a false positive.[10] The system, however, later did turn out to be an eclipsing binary, but one between a subgiant (B) and a brown dwarf (C) rather than between the red giant and a stellar companion.[3]

Brown dwarf

HD 190655 C was first named as a planetary candidate (KOI-3886.01) in 2012 as part of a collection of 150 promising transit signals. Early estimates gauged its radius at a colossal 21.70 R_🜨 (1.936 R_J),[11] which would have placed it among one of the largest exoplanets ever discovered. This was made even more intriguing by the fact that the object was projected to orbit a red giant (A) close to exceeding the Roche lobe, meaning it would have been a rare example of an enormous hot Jupiter in the last few million years of its life before falling into its host star.[3]

In 2021, however, meticulous observations revealed that the object instead orbited a subgiant companion star (B) rather than the red giant primary, and that it was causing radial velocity variations of ~7 km/s, far larger than initially thought, implying a mass much higher than the previous estimate of 1.81±0.11 M_J. Indeed, the mass of HD 190655 C is now thought to be at around 66 M_J, firmly placing it within the brown dwarf mass range.[3]

Due to its vicinity to the subgiant host, HD 190655 C receives 2,100 times as much radiative flux as Earth does. As a result, its atmosphere is puffed up to reach a radius of over 1.5 R_J, making it the most inflated known brown dwarf in 2021.[3]

See also

• Iota Draconis: a red giant similar to HD 190655 A, with two confirmed planets.
• WD 0032−317 b: another highly irradiated brown dwarf.
• Kepler-91b, K2-141b: planets close to their red giant hosts.

References

1. Brown, A. G. A.; et al. (Gaia collaboration) (2021). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics. 649: A1. arXiv:2012.01533. Bibcode:2021A&A...649A...1G. doi:10.1051/0004-6361/202039657. S2CID 227254300. (Erratum: doi:10.1051/0004-6361/202039657e). Gaia EDR3 record for this source at VizieR.
2. Zacharias, N.; et al. (14 January 2013). "The Fourth Us Naval Observatory CCD Astrograph Catalog (Ucac4)". The Astronomical Journal. 145 (2). American Astronomical Society: 44. arXiv:1212.6182. doi:10.1088/0004-6256/145/2/44. ISSN 0004-6256.
3. Lillo-Box, J.; et al. (2021). "Uncovering the ultimate planet impostor". Astronomy & Astrophysics. 653. EDP Sciences: A40. arXiv:2106.05011. doi:10.1051/0004-6361/202141158. ISSN 0004-6361.
4. "HD 190655". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 25 October 2024.
5. Jönsson, Henrik; et al. (17 August 2020). "APOGEE Data and Spectral Analysis from SDSS Data Release 16: Seven Years of Observations Including First Results from APOGEE-South". The Astronomical Journal. 160 (3). American Astronomical Society: 120. arXiv:2007.05537. doi:10.3847/1538-3881/aba592. ISSN 0004-6256.
6. Campante, Tiago L.; et al. (27 April 2023). "Revisiting the Red Giant Branch Hosts KOI-3886 and ι Draconis. Detailed Asteroseismic Modeling and Consolidated Stellar Parameters". The Astronomical Journal. 165 (5). American Astronomical Society: 214. arXiv:2304.01570. doi:10.3847/1538-3881/acc9c1. ISSN 0004-6256.
7. North, Gerald; et al. (2014). Observing Variable Stars, Novae and Supernovae. Cambridge University Press. p. 24. ISBN 978-1-107-63612-5.
8. Zarenski, Ed (2004). "Limiting Magnitude in Binoculars" (PDF). Cloudy Nights. Archived (PDF) from the original on 21 July 2011. Retrieved 6 May 2011.
9. Prša, Andrej; et al. (1 August 2016). "NOMINAL VALUES FOR SELECTED SOLAR AND PLANETARY QUANTITIES: IAU 2015 RESOLUTION B3 * †". The Astronomical Journal. 152 (2): 41. arXiv:1510.07674. doi:10.3847/0004-6256/152/2/41. ISSN 0004-6256.
10. Gaulme, P.; et al. (26 March 2013). "RED GIANTS IN ECLIPSING BINARY AND MULTIPLE-STAR SYSTEMS: MODELING AND ASTEROSEISMIC ANALYSIS OF 70 CANDIDATES FROM KEPLER DATA". The Astrophysical Journal. 767 (1). American Astronomical Society: 82. arXiv:1303.1197. doi:10.1088/0004-637x/767/1/82. ISSN 0004-637X.
11. Huang, X.; et al. (22 December 2012). "150 new transiting planet candidates from Kepler Q1–Q6 data". Monthly Notices of the Royal Astronomical Society. 429 (3). Oxford University Press (OUP): 2001–2018. arXiv:1205.6492. doi:10.1093/mnras/sts463. ISSN 1365-2966. VizieR Record for this source at VizieR.