Exocomets are plunging into HD 172555, a nearby young star surrounded by a debris disk, according to a team of astronomers led by Carol Grady of NASA’s Goddard Spaceflight Center and Eureka Scientific Inc. The comets were not seen directly around HD 172555, and the researchers inferred their presence when they used the NASA/ESA Hubble Space Telescope to detect gas that is likely the vaporized remnants of their icy nuclei.

HD 172555 is 95.2 light-years distant and is estimated to be 23 million years old.

Also known as HIP 92024, HR 7012, IRAS 18405-6455 and TYC 9077-2487-1, HD 172555 is a member of the Beta Pictoris Moving Group, a collection of stars born from the same stellar nursery.

Beta Pictoris, the stellar group’s namesake, also is feasting on exocomets traveling too close.

The group is important to study because it is the closest collection of young stars to our Solar System.

At least 37.5% of the more massive stars in the group either have a directly imaged exoplanet, such as 51 Eridani b in the 51 Eridani system, or infalling star-grazing bodies, or, in the case of Beta Pictoris, both types of objects.

“The grouping is at about the age that it should be building terrestrial planets,” Grady said.

In 2013, Dr. Flavien Kiefer of the Paris Institute of Astrophysics and co-authors discovered exocomets around HD 172555 in data from ESO’s HARPS planet-finding spectrograph.

The HARPS instrument detected the chemical fingerprints of calcium imprinted in the starlight, evidence that comet-like objects were falling into the star.

As a follow-up to that discovery, Grady and her colleagues used Hubble’s Space Telescope Imaging Spectrograph (STIS) and the Cosmic Origins Spectrograph (COS) in 2015 to conduct a spectrographic analysis in UV light, which allows Hubble to identify the signature of certain elements. Hubble made two observations, separated by six days.

Hubble detected silicon and carbon gas in the starlight. The gas was moving at about 360,000 miles per hour across the face of HD 172555.

The most likely explanation for the speedy gas is that Hubble is seeing material from exocomets that broke apart after streaking across the face of the star.

“We present HST STIS and COS spectra of HD 172555 which demonstrate that the star has chromospheric emission and variable infalling gas features in transitions of silicon and carbon ions at times when no Fe II absorption is seen in the UV data, and no Ca II absorption is seen in contemporary optical spectra,” the authors said.

“We discuss apparent optical depths for the infall features. The lack of CO absorption and stable gas absorption at the system velocity is consistent with the absence of a cold Kuiper belt analog in this system.”

“The presence of infall in some species at one epoch and others at different epochs suggests that, like Beta Pictoris, there may be more than one family of exocomets.”

According to the team, the gaseous debris from the disintegrating exocomets is vastly dispersed in front of HD 172555.

“As transiting features go, this vaporized material is easy to see because it contains very large structures,” Grady said.

“This is in marked contrast to trying to find a small transiting exoplanet, where you’re looking for tiny dips in the star’s light.”

Grady and co-authors reported their findings Jan. 6, 2017 at the 229th Meeting of the American Astronomical Society in Grapevine, Texas.

_____

C.A. Grady et al. The Transiting Exocomets in the HD 172555 System. 229th AAS Meeting, abstract # 345.12