# Groombridge 1618

Groombridge 1618
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Ursa Major
Right ascension 10h 11m 22.14051s[1]
Declination +49° 27′ 15.2567″[1]
Apparent magnitude (V) +6.60[2]
Characteristics
Spectral type K8 V[3]
U−B color index +1.27[2]
B−V color index +1.34[2]
Variable type BY Dra,[4] Flare star[5]
Astrometry
Proper motion (μ) RA: –1362.32[1] mas/yr
Dec.: –504.52[1] mas/yr
Parallax (π) 205.21 ± 0.54[1] mas
Distance 15.89 ± 0.04 ly
(4.87 ± 0.01 pc)
Details
Mass 0.670 ± 0.033[7] M
Luminosity 0.15[8] L
Surface gravity (log g) 4.70[7] cgs
Temperature 3,970[9] K
Metallicity [Fe/H] –0.03[9] dex
Rotational velocity (v sin i) 2.8[10] km/s
Age 6.6[8] Gyr
Other designations
BD+50 1725, GCTP 2390.00, GJ 380, HD 88230, HIP 49908, IRAS 10082+4942, LFT 696, LHS 280, LTT 12732, SAO 43223.[3]

Groombridge 1618 is a star in the constellation Ursa Major. It is located close to Earth, at a distance of less than 16 light years. It is an orange dwarf star of spectral type K8 V.

## Properties

This star was first identified as entry number 1618 in the work A Catalog of Circumpolar Stars, published posthumously by Stephen Groombridge in 1838.[11] It has a high proper motion across the sky, which is normally taken as an indicator that the star is located nearby—making it an early candidate for parallax measurement of its distance. In 1884 the parallax angle was measured as 0.322 ± 0″.023, which is larger than the modern value of 0″.205.[12]

Groombridge 1618 has a stellar classification of K8 V, which means it is a K-type main sequence star that is generating energy by fusing hydrogen at its core. It has 67% of the mass of the Sun, 61% of the Sun's radius,[7] but radiates only 4.6% of the Sun's energy. The effective surface temperature of the star's photosphere is about 4,000 K, giving it an orange hue.

It is a BY Draconis variable with a surface magnetic field strength of 750 G.[4] The chromosphere is relatively inactive[13] and possesses star spots comparable to Sun spots. However, like the star UV Ceti, it has been observed to undergo increases in luminosity as a flare star.[5] It has a greater luminosity than most flare stars, which are typically red dwarfs, but is less active. The level of activity suggests that this is a somewhat youthful star.

## System

A search for an excess amount of infrared emission from this star by the Infrared Space Observatory came up negative, implying that Groombridge 1618 does not possess a debris disk (such as Vega).[14] However, observations using the Herschel Space Observatory showed a small excess suggesting the presence of a low temperature debris disk. The data can be modeled by a ring of coarse, highly-reflective dust at a temperature below 22 K orbiting at least 51 AU from the host star.[8] If this star does have a companion, astrometric measurements appear to place an upper bound of 3–12 times the mass of Jupiter on such a hypothetical object (for orbital periods in the range of 5–50 years).[15]

According to Marcy & Benitz (1989),[16] a possible periodicity of 122 days has been detected, inferring the potential presence of a massive planetary object with minimum mass 4 times that of Jupiter. This candidate planet has not been confirmed and the signal the authors had found could have been due to intrinsic stellar activity from the star's young age. If confirmed, the planet would be located at the outer edge of the star's habitable zone.[note 1]

The unconfirmed Groombridge 1618 system[16]
Companion
(in order from star)
Mass Semimajor axis
(AU)
Orbital period
(days)
b (unconfirmed) ≥4 MJ 0.41 122 0

An examination of this system in 2010 using the MMT telescope fitted with adaptive optics failed to detect a planetary companion.[18]

## Notes

1. ^ $\scriptstyle S_{eff_{\ast}}= S_{eff_{\odot}} + aT_{\ast} + bT_{\ast}^2 + cT_{\ast}^3 + dT_{\ast}^4$ [17] when used to calculate the stellar flux reaching the outer atmosphere of an Earth-like planet orbiting Groombridge 1618 at the Outer Habitable Zone Edge - the Maximum Greenhouse limit gives a $\scriptstyle S_{eff_{\ast}}$ of $\scriptstyle{0.261}$ or $\scriptstyle{26.1%}$ the stellar ﬂux reaching the top of Earth's atmosphere. By applying the previously calculated stellar flux and the known $\scriptstyle{4.6%}$ luminosity of Groombridge 1618 into the equation, $\scriptstyle {d = {\left(\frac{{L_{\ast}}/{L_{\odot}}}{S_{eff_{\ast}}} \right)^{0.5} AU}}$,[17] the distance of the Outer HZ - Maximum Greenhouse limit from Groombridge 1618 can be calculated as $\scriptstyle{0.42 AU}$.

## References

1. van Leeuwen, F. (November 2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
2. ^ a b c Argue, A. N. (1966), "UBV photometry of 550 F, G and K type stars", Monthly Notices of the Royal Astronomical Society 133: 475, Bibcode:1966MNRAS.133..475A
3. ^ a b "NSV 4765 -- Flare Star". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2013-08-01.
4. ^ a b Gudel, M. (October 1992), "Radio and X-ray emission from main-sequence K stars", Astronomy and Astrophysics 264 (2): L31–L34, Bibcode:1992A&A...264L..31G.
5. ^ a b Andrillat, Y.; Morguleff, N. (1967), "Three potassium-flare stars", in Hack, Margherita, Proceedings of the Colloquium, held in Trieste, June 13-17, 1966, Trieste, Bibcode:1967lts..conf..160A.
6. ^ Holmberg, J.; Nordstrom, B.; Andersen, J. (July 2009). "The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics". Astronomy and Astrophysics 501 (3): 941−947. arXiv:0811.3982. Bibcode:2009A&A...501..941H. doi:10.1051/0004-6361/200811191.
7. ^ a b c d Ségransan, D.; Kervella, P.; Forveille, T.; Queloz, D. (January 2003). "First radius measurements of very low mass stars with the VLTI". Astronomy and Astrophysics 397: L5–L8. arXiv:astro-ph/0211647. Bibcode:2003A&A...397L...5S. doi:10.1051/0004-6361:20021714.
8. ^ a b c Eiroa, C. et al. (December 2011), "Herschel discovery of a new class of cold, faint debris discs", Astronomy & Astrophysics 536: L4, arXiv:1110.4826, Bibcode:2011A&A...536L...4E, doi:10.1051/0004-6361/201117797.
9. ^ a b Woolf, Vincent M.; Wallerstein, George (January 2005). "Metallicity measurements using atomic lines in M and K dwarf stars". Monthly Notices of the Royal Astronomical Society 356 (3): 963–968. arXiv:astro-ph/0410452. Bibcode:2005MNRAS.356..963W. doi:10.1111/j.1365-2966.2004.08515.x. See table 3.
10. ^ López-Morales, Mercedes (May 2007). "On the Correlation between the Magnetic Activity Levels, Metallicities, and Radii of Low-Mass Stars". The Astrophysical Journal 660 (1): 732–739. arXiv:astro-ph/0701702. Bibcode:2007ApJ...660..732L. doi:10.1086/513142.
11. ^ Dyson, F. W.; Thackeray, W. G.; Christie, W. H. M. (1905). "New reduction of Groombridge's catalogue of circumpolar stars". Proceeding of the Royal Society, Edinburgh. Bibcode:1905Gmb...C......0D.
12. ^ Ball, Robert S. (1884). "On the Annual Parallax of Groombridge 1618". Dunsink Observatory Publications 5 (2): 187–217. Bibcode:1884DunOP...5..187B.
13. ^ Byrne, P. B.; Doyle, J. G. (November 1990), "Activity in late-type stars. VII - Chromospheric and transition region line fluxes in 2 dM and 1 dM(e) stars", Astronomy and Astrophysics 238 (1–2): 221–226, Bibcode:1990A&A...238..221B.
14. ^ Laureijs, R. J. et al. (2002). "A 25 micron search for Vega-like disks around main-sequence stars with ISO". Astronomy & Astrophysics 387 (1): L285–L293. Bibcode:2002A&A...387..285L. doi:10.1051/0004-6361:20020366.
15. ^ Hershey, J. L.; Borgman, E. R. (1978). "Upper Limits on the Mass of a Dark Companion of Groombridge 1618 from the 40-year Sproul Plate Series". Bulletin of the American Astronomical Society, 10: 630. Bibcode:1978BAAS...10..630H.
16. ^ a b Marcy, Geoffrey W.; Benitz, Karsten J. (1989). "A search for substellar companions to low-mass stars". Astrophysical Journal, Part 1 344 (1): 441–453. Bibcode:1989ApJ...344..441M. doi:10.1086/167812.
17. ^ a b Kopparapu, R. K.; Ramirez, R.; Kasting, J.F.; Eymet, V.; Robinson, T. D.; Mahadevan, S.; Terrien, R.C.; Domagal-Goldman, S.; Meadows, R.; Deshpande, V. (March 2013). "Habitable Zones around Main-sequence Stars: New Estimates". The Astrophysical Journal 765 (2): 16. arXiv:1301.6674. Bibcode:2013ApJ...765..131K. doi:10.1088/0004-637X/765/2/131.
18. ^ Heinze, A. N. et al. (May 2010). "Constraints on Long-period Planets from an L'- and M-band Survey of Nearby Sun-like Stars: Observations". The Astrophysical Journal 714 (2): 1551–1569. arXiv:1003.5340. Bibcode:2010ApJ...714.1551H. doi:10.1088/0004-637X/714/2/1551.