A Peek Inside NGC 7538

The active star forming region NGC 7538. Image by Fred Calvert/Adam Block/NOAO/AURA/NSF

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Often overshadowed by the more famous Bubble Nebula which lies nearby, NGC 7538 is an exciting emission and reflection nebula located in Cepheus. While it is often overlooked by amateur astronomers, professionals looking to study stellar formation find it an exciting target as it is the host to ongoing star formation, including the largest known protostar.

Because of the dusty nature of this region, studies targeting the nebula are frequently conducted in longer wavelengths, ranging from the infrared to the radio. Previous studies have put the age of the forming stars at around ~1-4 million years and at a distance of ~2.8 kiloparsecs. Within it, several individual sub groups of star formation seem to have occurred. Among some of the more interesting individual forming stars are NGC 7538S and MM 1.

Observations from earlier this year targeted NGC 7538S. This protostar is embedded in a collapsing core of approximately 85 – 115 solar masses and hosts a rotating accretion disc as well as large outflows of material. Although the star has not finished forming, the conditions are right for it to form into a high mass B star and is undergoing accretion at an unusually high rate of 1/1000th of a solar mass per year.

More recently another paper explores several other forming stars in the region including the massive MM 1. This star is already estimated to have accumulated 20-30 solar masses and be well on the way to forming an O class star. But it’s not done yet. Radial velocity measurements of molecules in the protostar’s vicinity indicate it’s still undergoing large amounts of accretion, mostly from its equatorial plane. Numerous studies have shown that this massive star is creating powerful jets.

In addition, this new study identifies an additional eight cores forming into young stars near MM 1. These cores are interesting because they exist in regions where the density and temperature were not expected to be sufficiently high to induce star formation. This suggests that their formation was not uniquely due to a self induced collapse, but rather, triggered by shock waves or magnetic fields. Although no studies have searched for the signs of magnetic fields in the region, there are indications that numerous shock waves exist. Additionally, four of these cores have mass available to them similar to that of MM 1 which may allow them to form into a grouping of high mass stars similar to the famous Trapezium in Orion. These stars all exist in a narrowly confined region of about 1 light year, which is also similar to the separation of the Trapezium. Many of the newly discovered cores have large outflows and maser emission as well.

Further studies on this region will certainly uncover new protostars and assist astronomers in understanding how clusters of stars form. Already, astronomers have used it to help probe the Initial Mass Function which describes the number of stars forming for various masses. Additionally, with small clusters of stars like the Trapezium being common, catching one in the act of forming may help astronomers determine just how they form.

J-E-T-S, Jets, Jets, Jets!

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It seems oddly appropriate to be writing about astrophysical jets on Thanksgiving Day, when the New York football Jets will be featured on television. In the most recent issue of Science, Carlos Carrasco-Gonzalez and collaborators write about how their observations of radio emissions from young stellar objects (YSOs) shed light one of the unsolved problems in astrophysics; what are the mechanisms that form the streams of plasma known as polar jets? Although we are still early in the game, Carrasco-Gonzalez et al have moved us closer to the goal line with their discovery.

Astronomers see polar jets in many places in the Universe. The largest polar jets are those seen in active galaxies such as quasars. They are also found in gamma-ray bursters, cataclysmic variable stars, X-ray binaries and protostars in the process of becoming main sequence stars. All these objects have several features in common: a central gravitational source, such as a black hole or white dwarf, an accretion disk, diffuse matter orbiting around the central mass, and a strong magnetic field.

Relativistic jet from an AGN. Credit: Pearson Education, Inc., Upper Saddle River, New Jersey

When matter is emitted at speeds approaching the speed of light, these jets are called relativistic jets. These are normally the jets produced by supermassive black holes in active galaxies. These jets emit energy in the form of radio waves produced by electrons as they spiral around magnetic fields, a process called synchrotron emission. Extremely distant active galactic nuclei (AGN) have been mapped out in great detail using radio interferometers like the Very Large Array in New Mexico. These emissions can be used to estimate the direction and intensity of AGNs magnetic fields, but other basic information, such as the velocity and amount of mass loss, are not well known.

On the other hand, astronomers know a great deal about the polar jets emitted by young stars through the emission lines in their spectra. The density, temperature and radial velocity of nearby stellar jets can be measured very well. The only thing missing from the recipe is the strength of the magnetic field. Ironically, this is the one thing that we can measure well in distant AGN. It seemed unlikely that stellar jets would produce synchrotron emissions since the temperatures in these jets are usually only a few thousand degrees. The exciting news from Carrasco-Gonzalez et al is that jets from young stars do emit synchrotron radiation, which allowed them to measure the strength and direction of the magnetic field in the massive Herbig-Haro object, HH 80-81, a protostar 10 times as massive and 17,000 times more luminous than our Sun.

Finally obtaining data related to the intensity and orientation of the magnetic field lines in YSO’s and their similarity to the characteristics of AGN suggests we may be that much closer to understanding the common origin of all astrophysical jets. Yet another thing to be thankful for on this day.