V1400 Centauri

The star has been catalogued in as early as the 1990s by the Hubble Guide Star Catalog, which identified the star and measured its position in a pair of photographic plates taken in 1974 and 1979. The star has been catalogued by other sky surveys, including the All Sky Automated Survey (ASAS), Two Micron All-Sky Survey (2MASS), Super Wide Angle Search for Planets (1SWASP), and the Wide-field Infrared Survey Explorer (WISE). Typically in these catalogues, the star is given designations such as 1SWASP J140747.93–394542.6, which comprises the survey name followed by the star's location in equatorial coordinates. As such designations can be unwieldy, researchers simply call the star "J1407". The star was given the official variable star designation V1400 Centauri in 2015, when it was added to the International Astronomical Union's General Catalogue of Variable Stars. A 2018 research paper on stars with unusual dimming periods nicknamed V1400 Centauri "Mamajek's Object", after the astronomer Eric Mamajek who identified the star's unusual dimming in 2007. == Stellar properties ==

Location and age V1400 Centauri is located in the constellation Centaurus, about 40 degrees south of the celestial equator. The most recent parallax measurements by the Gaia spacecraft indicate V1400 Centauri is located from the Sun. Observations of V1400 Centauri's position over time have shown that it has a southwestward proper motion consistent with that of the Scorpius–Centaurus association, an OB association of young stars with ages between 11–17 million years and distances between from the Sun. The Scorpius–Centaurus association is the nearest OB association to the Sun, and is believed to have formed out of a molecular cloud that has since been blown away by the stellar winds of the association's most massive stars. V1400 Centauri is closest to the Upper Centaurus–Lupus subgroup of the Scorpius–Centaurus association, which has an age range of 14–18 million years and distance range of . Given V1400 Centauri's similar distance and proper motion, it very likely belongs to the Scorpius–Centaurus association, which would mean it must be a young star within the age range of the Upper Centaurus–Lupus subgroup. A 2012 estimate of V1400 Centauri's age assumes it is equal to 16 million years, the mean age of the Upper Centaurus–Lupus subgroup, while a 2018 estimate from Gaia measurements puts the star's age at million years. Spectral type and physical characteristics V1400 Centauri is a pre-main sequence star of spectral class K5 IVe Li. "K" means V1400 Centauri is an orange K-type star, and the adjoining number "5" ranks V1400 Centauri's relative temperature on a scale of 9 (coolest) to 0 (hottest) for K-type stars. V1400 Centauri is given the subgiant luminosity class "IV", because it has a brighter luminosity than K-type main-sequence stars (luminosity class V). The letter "e" indicates V1400 Centauri exhibits weak hydrogen-alpha emission lines in its visible light spectrum. Lastly, "Li" indicates V1400 Centauri is abundant in lithium. Measurements from the Gaia spacecraft's third and most recent data release (Gaia DR3) indicate V1400 Centauri is about 7% larger than the Sun in radius (), but is slightly less massive than the Sun. Depending on whether magnetic effects are taken into account in V1400 Centauri's stellar evolution or not, the star's mass could be either or , respectively. Young stars tend to be magnetically active, and neglecting their magnetic effects results in an underestimation of their mass. An older estimate of V1400 Centauri's mass from Gaias second data release (Gaia DR2) in 2018 gives , but does not take magnetic effects into account. V1400 Centauri is cooler and less luminous than the Sun, with an effective temperature of about and a luminosity about 34% that of the Sun. V1400 Centauri has an estimated surface gravity of about (over 20 times the gravity of Earth), based on Gaia measurements of the star's brightness, distance, and color. Gaia measurements also indicate V1400 Centauri has a lower metallicity than the Sun. Viewed from Earth, V1400 Centauri appears marginally redder than a typical K5-type star due to light extinction by interstellar dust between Earth and the star. The star does not exhibit excess thermal emission in near- and mid-infrared wavelengths and lacks strong emission lines in its spectrum, which indicates it lacks a substantial accretion disk or protoplanetary disk. Rotation and variability Like most young stars, V1400 Centauri rotates rapidly with a rotation period of approximately 3.2 days. The rapid rotation of V1400 Centauri strengthens its magnetic field via the dynamo process, which leads to the formation of starspots on its surface. As V1400 Centauri rotates, its starspots come into and out of view, causing the star's brightness to periodically fluctuate by 5%, or about 0.1 magnitudes in amplitude. The star's rotation period varies by 0.02 days over a 5.4-year-long magnetic activity cycle, due to the long-term movement of starspots across the star's differentially rotating surface. V1400 Centauri is known to emit soft X-rays due to its corona being heated by its rotationally-strengthened magnetic field. Because of its young age, starspot variability, and lack of dust accretion, V1400 Centauri is classified as a weak-lined T Tauri variable. Spectroscopic measurements of Doppler broadening in V1400 Centauri's spectral absorption lines indicate the star has a projected rotational velocity of . Given V1400 Centauri's rotation period, radius, and temperature, the star's true equatorial rotation velocity is , which indicates that the star's rotation axis is inclined with respect to Earth's line of sight. == 2007 dimming events ==

of V1400 Centauri's brightness during 2007 shows a 56-day-long series of dimming events. It has been hypothesized that these dimmings were caused by a large disked object (J1407b) eclipsing the star. During 7 April to 4 June 2007, telescopes of the Super Wide Angle Search for Planets (SuperWASP) and All Sky Automated Survey (ASAS) projects recorded V1400 Centauri undergoing a series of significant dimming events for 56 days. The pattern of these dimming events was complex yet nearly symmetrical, suggesting they were caused by an opaque, disk-like structure eclipsing the star. The object thought to be responsible for these eclipses has been dubbed J1407b. V1400 Centauri's 2007 dimmings were discovered on 3 December 2010 by Mark Pecaut, who was a graduate student of Eric E. Mamajek at the University of Rochester. Mamajek, Pecaut, and collaborators announced the discovery in 2012. Mamajek's team initially hypothesized that J1407b was a ringed exoplanet orbiting the star, but this explanation turned out to be unlikely because no new eclipses were ever observed, nor did historical photographic plates show any evidence for eclipses in the last century. Astronomers have alternatively proposed that J1407b might have been a free-floating substellar object with a circumstellar disk that coincidentally eclipsed V1400 Centauri, with follow-up observations suggesting that its disk is not thick and dusty. == See also ==