Clouds on a Young Planet: First Science Results with FIRE

A UKIDSS false color image of the Ross 458 system, composed of a pair of M dwarfs (bright star in upper left corner) and the planetary-mass brown dwarf Ross 458C (circled in lower right corner).

To study the atmospheres of young planets outside our Solar System, we need not look  far.  The first brown dwarf science with the newly-commissioned FIRE spectrograph has revealed the presence of rock clouds in the atmosphere of a planetary-mass companion to the nearby Ross 458 system.  The presence of these clouds, and the planetary nature of the source, defy prior expectations.

The source in question is Ross 458C, a brown dwarf candidate identified in the UKIDSS survey in early 2010 by two independent studies led by Drs. Rolf-Dieter Scholz and Betrand Goldman.  This source, also known as ULAS J130041.72+122114.7, is located 1.7′ (0.028 degrees) southeast of the Ross 458 system, a pair of magnetically active M dwarfs only 11.2 pc (36.5 light-years) from the Sun. The colors and faintness of Ross 458C, and that fact that it co-moves with the Ross 458 system, led both studies to conclude that it was potentially a very cool and very low-mass brown dwarf companion.  However, neither study had the necessary spectral data to probe its atmosphere.

Enter the Folded-port Infrared Echellette, or FIRE, spectrograph, which was commissioned on the Magellan 6.5m Baade telescope in late February 2010 (see this prior blog post ).  One of the first targets we aimed FIRE at was Ross 458C, in order to determine the nature of this interesting faint object.  The resulting spectrum revealed strong bands of water and methane gas, typical of low-temperature brown dwarfs of the T spectral class.  We classified Ross 458C as a T8 dwarf, found its luminosity to be 400,000 times fainter than the Sun, and estimated its surface temperature to be a mere 650 °K (710 °F) – about the temperature of a pizza oven.  The rapid rotation and strong magnetic activity of Ross 458A indicates that the system is relatively young – a mere 150-800 million years old – which would give Ross 458C a mass of only 6-11 times that of Jupiter based on evolutionary models.  This is similar to the masses of the planets found orbiting the HR 8799 system.

FIRE spectrum of Ross 458C (black line) compared to best-fit atmosphere models with (blue line) and without (red line) clouds. The cloudy model provides a better fit, evidence that clouds are present in the atmosphere of this source.

Analysis of the FIRE spectrum of Ross 458C revealed another surprise – its atmosphere appears to be covered in clouds.  Comparing the spectrum to models created by Drs. Mark Marley and Didier Saumon, we found that the best fits could be made only when the absorption effects of clouds were included in the models.  The presence of these clouds – which are made of rocks and metal rather than water – was unexpected, as it is has long been assumed that such clouds are sunk deep below the atmospheres of T-type brown dwarfs. However, the young age of Ross 458C may allow these clouds to reside at higher layers in the atmosphere and form larger particles, in both cases making them easier to detect.

Ross 458C not only challenges our understanding of clouds in brown dwarf atmospheres, it challenges our very definition of what a brown dwarf is.  As a non-fusing object in orbit around a pair of stars, and with a mass below the limit of even deuterium fusion (about 13 Jupiter masses), it satisfies all of the requirements for a “planet” as laid out by the International Astronomical Union in 2006 and in an article published by Drs. Gibor Basri and Mike Brown, also in 2006.  Yet this is certainly a very unusual planet, since it has an orbit that is roughly 1000 AU in radius and is at least 6 times more massive and hotter than Jupiter.  As in many scientific endeavors, the more we learn, the more we find our assumptions and expectations might be wrong!

This result was published in in the Astrophysical Journal, and presented at the 16th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun and the 217th American Astronomical Society Meeting. Authors include Adam J. Burgasser (UCSD), Robert A. Simcoe (MIT), John J. Bochanski (MIT), Didier Saumon (LANL), Eric E. Mamajek (U. Rochester), Michael C. Cushing (NASA/JPL), Mark S. Marley (NASA/Ames), Craig McMurtry (U. Rochester), Judith L. Pipher (U. Rochester) and William J. Forrest (U. Rochester)

Discovery of the Nearest Young Binary

Three-color image of the sky around TWA 30, with the faint red companion TWA 30B indicated due north.

Earlier this year, U. Hawaii Graduate student Dagny Looper reported the discovery of a young, low-mass and highly active star in the TW Hydrae Association, TWA 30.  She has now found that that star is just one component of an unusual pair.

Only 80″ (0.02 degrees) north of TWA 30 lies a faint red star just barely visible in the image above.  This source is so red that it was worth a look with the MagE spectrograph at the Magellan Telescopes.  The resulting spectrum revealed a rich set of the same forbidden emission lines the we saw in the spectrum of TWA 30, indicating accretion onto the star from a circumstellar disk and  powerful magnetically-driven jets emanating from the star’s poles.  Moreover, we found two lines of forbidden neutral carbon emission, [C I], an important probe of the gas in the disk that surrounds this source, according to a 2009 study led by Dr. Barbara Ercolano.  This is the first time these lines have been seen in the spectrum of a young star.

Two spectra (black and red lines) of TWA 30B taken with the MagE spectrograph, highlighting the bevy of emission lines arising from accretion onto and jets powered by this young star. The forbidden carbon lines are seen in the bottom panel around 8700 A and 9800 A

Like TWA 30, the new source is highly variable in the near-infrared, changing its color by over a  magnitude over the course of a few days.  However, its “direction” of variability on a near-infrared color-color plot is different from that of TWA 30, indicating that it is caused not by changes of line-of-sight reddening, but changes in how its disk is blocking starlight.  The disk, apparently viewed edge-on, has structure to it!

Variations in the near-infrared colors of TWA 30A (red stars) and TWA 30B (green stars). TWA 30A's changes are consistent with reddening caused by dust extinction (arrow at lower right), but TWA 30B's variations appear to be due to patchy obscuration by a circumstellar disk.

The motion of the new source both across the sky and radially away from us, is consistent with being both a member of the TW Hydrae association and a co-moving companion to TWA 30.  Hence, these two stars – TWA 30A and TWA 30B – are a remarkable wide young binary system, nearly 3400 AU in projected separation.  What is most surprising is that TWA 30B is about 5 magnitudes fainter than TWA 30A in the near-infrared, even though it has an earlier spectral type.  This is further evidence that the star is being obscured by an edge-on disk, much like the faint secondary in the HK Tau B binary star system.  Upcoming Hubble Space Telescope observations led by Dr. John Bochanski should reveal more about this remarkable system.

This result was published in in the Astronomical Journal. Authors include Dagny L. Looper (U. Hawaii), John J. Bochanski (MIT/Penn. State), Adam J. Burgasser (UCSD), Subhanjoy Mohanty (Imperial College), Eric E. Mamajek (U. Rochester), Jacqueline K. Faherty (SUNY Stony Brook), Andrew A. West (Boston U.), and Mark A. Pitts (U. Hawaii).

An Unsuspecting Pair

l-and-t-binary-brown-dwarf-system

Found two brown dwarfs for the effort of one

Spitzer Science Center colleague Christopher Gelino and I report the identification of low-mass binary system that refuses to show itself.  The source, 2MASS J20261584–2943124, is an L dwarf which until now had seemed to be a perfectly unassuming source.  However,  low-resolution, near-infrared spectroscopy we obtained with the IRTF SpeX spectrograph revealed a peculiar absorption feature that is commonly seen in very low-mass “spectral binaries”, blends of stars with different spectral types.  Our analysis indicates that this source is an L dwarf plus T dwarf pair, with a relative brightness of roughly 4 magnitudes (or 40 times fainter) in the near-infrared.  We were unable to resolve the putative pair with the Keck Observatory laser guide star adaptive optics system, which rules a binary wider than  9 Astronomical Units (about the distance between the Sun and Saturn).  Next step: look for Doppler shifts in the spectrum that would indicate the gravitational influence of the unseen companion and allow us to measure its mass.

This research was published in the Astronomical Journal. Authors include Christopher R. Gelino (Spitzer Science Center) and Adam J. Burgasser (UCSD).

July 2010

A Blue Brown Dwarf

Not all brown dwarfs are brown

We report observations of an unusually blue brown dwarf, a nearby object that may be among the coldest and oldest brown dwarfs known.  The source, ULAS J141623.94+134836.3, was originally discovered in the UKIDSS survey independently by R. Scholz and B. Burningham et al., and early results indicated its surface could be as cool as 500 K. It could even be the first Y dwarf.   Our near-infrared spectrum, obtained with the IRTF SpeX spectrograph, instead shows it to be somewhat warmer (650 K), as well as old, massive and depleted in “metals” (any element other than hydrogen and helium).  ULAS J1416+1348 is also a companion to the unusually blue L dwarf SDSS J141624.08+134826.7 discovered earlier this year by several groups.  This nearby brown dwarf pair has generated a lot of interest among astronomers, with five publications in six months.

This result was published in in the Astronomical Journal; it was also an IRTF science highlight.

June 2010

Discovery of a Wacko Star

Jets, disks and accretion in nearby low mass star

U. Hawaii Graduate student Dagny Looper reports the discovery of a young, low-mass nearby star that is both unusually active and highly variable. The star, TWA 30, is a member of the TW Hydrae Association, roughly two dozen ~8 million year-old stars located about 50 pc away. TWA 30 is the newest member of this group, and one of the most intriguing. Its optical spectrum shows classical and forbidden emission lines, indicating that it is both accreting material and emitting high-speed jets of gas. The star’s near-infrared color also varies dramatically on week-long timescales, evidence that it periodically hides behind a nearly edge-on circumstellar disk. This makes TWA 30 one of the nearest T Tauri stars to the Sun.

The paper was published in the Astrophysical Journal.

May 2010