HH 1177: a Massive Rising Star Outside the Milky Way

In recent years, thanks also to the development of new technologies for astronomical observations, an increasing number of rotating structures or accretion disks around other stars have been discovered. All of them, however, are located inside our Galaxy, the Milky Way. These kinds of structures astronomers believe are where planets form. New observations of the HH 1177 system done with the Atacama Large Millimeter/submillimeter Array (ALMA), however, are about to change the cards on the table.

Rotating structures: where to find them?

Usually, rotating structures are associated with young stellar objects (YSO), and only recently have astronomers started to detect them around massive YSOs (MSYO). This lack of observations is due to the fast timescale on which MSYOs evolve: the star goes through the accretion phase before being detectable (it is hidden by the dense molecular cloud within which they are born), making this phase difficult to study. In addition to that, even the presence of a disk is hard to find outside of our Galaxy due to the limited resolution of both ground and space-based observatories.

In 2019 ESO announced that the Very Large Telescope (VLT), thanks to the Multi Unit Spectroscopic Explorer (MUSE), spotted a jet from a forming star inside the Large Magellanic Cloud, a small galaxy near the Milky Way. This system was then called HH 1177 and represented an important discovery for two reasons. The first one is that it is the first (and for now only) MYSO optically identified. As for the second one, jets like the one associated with HH 1177 are generally taken as very strong proof for ongoing disk accretion leading it to be an ideal target for their research outside our galaxy.

ALMA peers inside extragalactic clouds

Here comes ALMA, able to perform the high sensitivity and angular resolution observations needed to detect circumstellar gas in extragalactic MYSOs. To confirm the idea they had about HH 1177, Astronomers estimated the gas velocity to be around 225 km/s close to the center. Combined with the extrapolated kinematic, this was the main proof confirming all the hypotheses.

Its discovery is crucial since it allows scientists to study how the formation of massive stars might differ in a galaxy different from ours, with different amounts of metal and dust contents. One of the consequences, as astronomers say, could be the high stability of the inner region of the system studied. The scarcity of these materials, indeed, favored internal irradiation, maintaining a high disk temperature and preventing it from fragmentation.

ALMA & MUSE: instruments with superpowers

This important discovery has been possible thanks to two important machines.
The first one is the Multi Unit Spectroscopic Explorer (MUSE). It is a spectrograph that can study an entire object with just one observation. It was realized to study the Universe when the first galaxies and stars were forming and to map dark matter distribution. MUSE is installed at the VLT at the Paranal Observatory.

The second instrument is, actually, a group of 66 antennas. We are speaking about the Atacama Large Millimeter Array (ALMA). It has 54 antennas with a diameter of 12 meters, while the remaining have a diameter of 7 meters. ALMA is used just like a normal telescope, the only difference is the wavelength observed. Indeed, traditional telescopes capture visible light. Each of the antennas, instead, can study light at millimetric wavelengths and even submillimetric. They can also be combined to simulate a much bigger radio telescope.

While MUSE is owned by ESO, ALMA is a partnership between ESO and many international partners like the National Science Foundation National Astronomical Observatory of Japan (NAOJ).