NASA Astrobiology Institute (NAI)

High-energy photons in the young stellar environment are known to be important in stimulating chemical reactions of molecules and producing pre-biotic materials that might later be incorporated into comets. Observational tests are sorely needed to assess the significance of such processing for Astrobiology and to guide development of theoretical models for chemical evolution in proto-planetary environments. We have observed X-ray emission from young low- and intermediate- mass stars mainly using the Chandra and XMM-Newton satellites, to understand the short-term X-ray variation and long-term activity evolution. In this reporting period, we studied X-ray and radio activities of individual stars in a binary protostar system and found no apparent connection between the X-ray and radio variability. We observed X-ray emission from the intermediate-mass young star MWC480, which showed stronger absorption than expected from far UV data.

Project Title: X-ray emission from an intermediate-mass young star, protostar binary system and star-forming regions
Project Investigators: Dr. Robert Petre and Dr. Kenji Hamaguchi

High-energy photons in the young stellar environment are known to be important in stimulating chemical reactions of molecules and producing pre-biotic materials that might later be incorporated into comets. Observational tests are sorely needed to assess the significance of such processing for Astrobiology, and to guide development of theoretical models for chemical evolution in proto-planetary environments. In this reporting period, we approached this problem from two directions: study of the mechanism and history of X-ray emission from young low- and intermediate-mass stars, and study of hot plasma in massive star-forming regions with thousands of low-mass young stars.

Using Chandra and VLA data, we clearly resolved X-ray and radio emission from individual stars in the Class I protostar binary system IRS 5 in the R Corona Australis star forming region^1,2,3. The binary system, with projected separation less than 200 AU, has been the closest among protostar binary systems resolved in X-ray imagery (Figure 1). Our analysis revealed that the star that had shown X-ray flares (IRS 5a) was not the star with strongly variable cm continuum radio emission with significant circular polarization, which was IRS 5b. This result questioned the hypothesis that gyrosynchrotron radio emission seen from young stellar objects originates in high-energy electrons produced by X-ray flares and trapped in circumstellar magnetic tubes. The study also suggested structural and/or geometrical variations of the circumstellar magnetic field between protostars of similar ages.

We continuously teamed with GCA member Carol Grady in the GSFC Exoplanets Laboratory to study X-ray activity of young intermediate-mass stars4. Our most recent target is the actively accreting Herbig Ae star MWC 480, which drives a bipolar jet, and which has a massive disk with some of the best detailed chemical inventory of any of the protoplanetary disks. We studied our Chandra X-ray data of MWC 480, in line with HST coronagraphic imagery and FUSE FUV spectra. In contrast to other, jet-driving Herbig Ae stars, the soft X-rays were heavily absorbed in this system, with ^NH =5.7e21 cm^-2 (Figure 2). The absorption was an order of magnitude higher than expected for a similarly lightly reddened star and consistent with NH predictions for accretion columns in funneled accretion scenarios. This result suggests that the X-ray producing region in MWC 480 lies below such a column, viewed from between 18 and 50 degrees from the stellar rotational pole.

We also continuously studied extended hot plasma emission from the massive star-forming Carina nebula, where numerous low- and intermediate-mass stars as well as very massive stars such as Eta Carinae are forming⁵. In our Suzaku and XMM-Newton X-ray observations, the eastern tip of the Carina nebula had similar spectrum to the southern part of the Carina nebula6, suggesting that diffuse plasmas in both regions were produced by the same origin, perhaps explosions of one or multiple supernovae (Figure 3).

To further study evolution of X-ray activity in young low- and intermediate-mass stars, we take part in a Chandra Very Large Observing Project to map the Carina nebula, thereby to understand the structure of the diffuse emission and X-ray activity of young stars in the field.