V1400 Centauri

V1400 Centauri (known under its SuperWASP catalogue entry 1SWASP J140747.93−394542.6, or simply J1407) is a young, pre-main-sequence star that was eclipsed by a likely free-floating substellar object with a circumstellar disk or rings (known as J1407b or Mamajek's Object ) in April–May 2007. With an age around 20 million years, the star is about as massive as the Sun and is located in the constellation Centaurus at a distance of 451 light-years away from the Sun. V1400 Centauri is a member of Upper Centaurus–Lupus subgroup of the Scorpius–Centaurus association, a group of young, comoving stars close to the Sun.

The discovery of J1407b and its 2007 eclipse of V1400 Centauri was announced in 2012 by a team of astronomers led by Eric E. Mamajek, who directed an analysis of photometric data from the Super Wide Angle Search for Planets (SuperWASP) sky survey. Mamajek's team hypothesized that J1407b is a substellar object that could either be orbiting the star as a planet or binary companion, or is a gravitationally unbound (free-floating) object that coincidentally passed in front of the star. Later studies have since found evidence disfavoring the bound companion hypothesis: V1400 Centauri showed no eclipses after 2007 nor during the past 100 years, which suggests that J1407b does not orbit the star.

High-resolution imaging by the Atacama Large Millimeter Array (ALMA) in 2017 revealed a single faint object near V1400 Centauri, which could either be a young substellar object surrounded by a circumstellar disk, or a background galaxy. This faint object is far enough away from V1400 Centauri that it cannot be gravitationally bound to the star, which makes it a very likely candidate for J1407b. However, the faint object has only been observed once, so it is not yet confirmed whether it is a moving foreground object or a stationary background galaxy. If this faint object is J1407b, then it would have a mass below 6 Jupiter masses, which would also make it a sub-brown dwarf or a rogue planet.

Name and catalogue history
The star was first catalogued in the 1990s by the Hubble Guide Star Catalog, which found 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.

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 450.8 ± 0.9 ly 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 118-145 pc 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 115-141 pc. 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 $12.2$ 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 (1.07 solar 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 $5.904$ or $−23.108$, 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 Gaia's second data release (Gaia DR2) in 2018 gives $−21.048$, but does not take magnetic effects into account.

V1400 Centauri is cooler and less luminous than the Sun, with an effective temperature of about 4300 K and a luminosity about 34% that of the Sun. V1400 Centauri has an estimated surface gravity of about $0.977 solar mass$ (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.

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.

Spectroscopic measurements of Doppler broadening in V1400 Centauri's spectral absorption lines indicate the star has a projected rotational velocity of $0.891 solar mass$. Given V1400 Centauri's rotation period, radius, and temperature, the star's true equatorial rotation velocity is $0.95$, which indicates that the star's rotation axis is inclined $1.066$ with respect to Earth's line of sight.

Because of its young age, starspot variability, and lack of dust accretion, V1400 Centauri is classified as a weak-lined T Tauri variable. The star underwent a series of deep eclipse-like dimmings in 2007, which have been ascribed to the coincidental transit of J1407b's circumstellar disk. The star shows no signs of periodic dimming caused by transiting planets larger than Jupiter.

2007 eclipse by J1407b
During April–May 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, each lasting several days. The pattern of these dimming events was complex yet nearly symmetrical, indicating they were caused by a disk-like structure eclipsing V1400 Centauri. The light curve of V1400 Centauri during 2007 shows at least five major dimming events, including a very long and deep central eclipse bracketed by two pairs of shorter eclipses symmetrically occurring 12 days and 26 days before and after the deep eclipse midpoint. The deep eclipse lasted about 14 days (7 days from midpoint) and caused V1400 Centauri to dim by at least 3.3 magnitudes, or fall below 4.8% of its original brightness. The short eclipses before and after the deep eclipse were shallower and caused the star to dim by at least 1 magnitude, or fall below 40% of the star's original brightness.

A team of astronomers led by Eric E. Mamajek discovered the 2007 eclipse of V1400 Centauri while they were investigating SuperWASP's photometric data. Mamajek's team originally intended to use the SuperWASP data to validate candidate low-mass stars of the Scorpius–Centaurus association, which they had been studying since 2009. Mamajek's team presented their discovery of V1400 Centauri's eclipse in January 2012 at the 219th American Astronomical Society conference in Austin, Texas, and then formally published their results in The Astronomical Journal in March 2012.

Because V1400 Centauri does not emit excess infrared thermal radiation, the object that eclipsed the star must be substellar in mass, which means it could either be a brown dwarf or a planet. Mamajek's team hypothesized that this substellar object could either be orbiting V1400 Centauri as a planet or binary companion, or is a gravitationally unbound object that coincidentally passed in front of the star. The substellar object was first dubbed "J1407b" in a paper published by Tim van Werkhoven, Matthew Kenworthy, and Eric Mamajek in 2014, which assumed the object was orbiting V1400 Centauri.

Bound companion hypothesis


Mamajek's team initially considered the bound companion hypothesis plausible because there are known eclipsing binary stars where one component has a circumstellar disk (for example, Epsilon Aurigae), and V1400 Centauri is young enough that a protoplanetary disk could exist around the star and its putative companion.

J1407b has been popularly called a "Super Saturn" or a "Saturn on steroids" due to its massive system of circumplanetary rings with a radius of approximately 90 million km (0.6 AU). The orbital period of J1407b is estimated to be around a decade (3.5 to 13.8 years or 3825 days), and its most probable mass is approximately 13 to 26 Jupiter masses which would make the companion a brown dwarf but with considerable uncertainty. The ringed body can be ruled out as being a star with mass of over 80 Jupiter masses at greater than 99% confidence. The ring system has an estimated mass similar to that of the Earth. A major gap in the rings at about 61 million km (0.4 AU) from its center is considered to be indirect evidence of the existence of a shepherd exomoon with mass up to 0.8 Earth masses, but the exomoon hypothesis was challenged by a 2019 study.

J1407b has not been observed since its transit in 2007, which could be interpreted as the object being in a highly eccentric orbit around the star. Such an orbit could disrupt the ring system of J1407b. Dynamical simulations run by astronomers Steven Rieder and Matthew Kenworthy indicate that in order for J1407b's ring system to be stable, the rings must orbit J1407b in a retrograde motion, opposite to the direction J1407b orbits its host star. This retrograde solution for the ring system of J1407b allows for longer ring lifetimes as well as further constraints to the age of the ring system. The rings may be replenished over timescales as a result of processes that produce additional debris around J1407b, such as the tidal disruption of comets.

No additional transits from Jupiter-sized or larger planets were discovered in a 21-year long observational series by 2023.

Unbound object hypothesis
Issues with the stability of any rings combined with the lack of detection of another eclipse, suggests that J1407b may not be bound to J1407. No other deep eclipses has been found in the data spanning from 1890 to 1990, nor in recent time-series photometry from 2012-2018. A significant proportion of orbital periods for J1407b from 5 to 20 years can be disregarded, therefore if there is an actual orbital period it is likely outside of this range.

High-resolution imaging of V1400 Centauri in millimeter radio frequencies by the Atacama Large Millimeter Array (ALMA) in 2017 found no evidence of a bound companion $0.343$ in mass, but did detect a point source consistent with an unbound, $4.302$ substellar object surrounded by a disk of warm dust. This may be the object which transited V1400 Centauri in 2007, or it may be a background galaxy or an image artifact, which is considered less likely.