Groombridge 1618

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Groombridge 1618 is a star in the northern constellation Ursa Major. With an apparent visual magnitude of +6.6, it lies at or below the threshold of stars visible to the naked eye for an average observer. It is relatively close to Earth, at Template:Convert. This is a main sequence star of spectral type K7.5 Ve, having just 67% of the Sun's mass.

PropertiesEdit

This star was first identified as entry 1618 in the work A Catalog of Circumpolar Stars by Stephen Groombridge published posthumously in 1838.<ref name=prse1905>Template:Cite journal</ref> Its large proper motion across the sky suggested that it was relatively nearby and made it an early candidate for parallax measurements. In 1884 the parallax angle was measured as Template:Nowrap, which is larger than the modern value of 0″.205.<ref>Template:Cite journal</ref>

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,<ref name=aaa397_L5/> but radiates only 15% of the Sun's energy and only 4.6% of the Sun's energy in the visible light spectrum. 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.<ref name="AAA264_2_L31"/> The chromosphere is relatively inactive<ref name="AAA238_1_221"/> and produces star spots comparable to Sun spots. However, like UV Ceti, it has been observed to undergo increases in luminosity as a flare star.<ref name="Andrillat_Morguleff"/>

Search for planetsEdit

A search for excess infrared emission from this star by the Infrared Space Observatory came up negative, implying that Groombridge 1618 does not possess a nearby debris disk (such as Vega does).<ref name=AAA387_1_L285/> However, observations using the Herschel Space Observatory showed a small excess suggesting 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.<ref name="AAA536_L4"/> 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).<ref name="BAAS10_630"/>

Observations collated by Marcy & Benitz (1989),<ref name="Marcy & Benitz">Template:Cite journal</ref> tend towards a single notable object with periodicity of 122 days as a planetary object with minimum mass 4 times that of Jupiter. This candidate planet was never confirmed and the signal the authors had found could have been due to intrinsic stellar activity. If confirmed, the planet would be within the star's habitable zone.<ref><math>\scriptstyle S_{eff_{\ast</ref>= S_{eff_{\odot}} + aT_{\ast} + bT_{\ast}^2 + cT_{\ast}^3 + dT_{\ast}^4</math> <ref name=kopparapu-2013/> when used to calculate the stellar flux reaching the outer atmosphere of an Earth-like planet orbiting Groombridge 1618 at the Inner Habitable Zone edge - the Runaway Greenhouse limit gives a <math>\scriptstyle S_{eff_{\ast}}</math> of 0.9397 or 93.97% the stellar flux reaching the top of Earth's atmosphere. By applying the previously calculated stellar flux and the known 15% luminosity of Groombridge 1618 into the equation, <math>\scriptstyle {d = {\left(\frac{{L_{\ast}}/{L_{\odot}}}{S_{eff_{\ast}}} \right)^{0.5} AU}}</math>,<ref name=kopparapu-2013/> the distance of the Inner HZ - Runaway Greenhouse limit from Groombridge 1618 can be calculated as <math>\scriptstyle{0.4 AU}</math>.|group="note"|name=maximum greenhouse}}

An examination of this system in 2010 using the MMT telescope fitted with adaptive optics failed to detect a planetary companion.<ref>Template:Cite journal</ref>

The habitable zone for this star, defined where liquid water could be present on an Earth-like planet, is at a radius of 0.26–0.56 AU, where 1 AU is the average distance from the Earth to the Sun.<ref name="aaa555_A104"/>

The star is among five nearby K-type stars of a type in a 'sweet spot’ between Sun-analog stars and M stars for the likelihood of evolved life, per analysis of Giada Arney from NASA’s Goddard Space Flight Center.<ref name=Nasa2019-03-07/>