Diagnosing Mass Flows Around Herbig Ae/Be Stars
Cauley, Paul Wilson
Johns-Krull, Christopher M
Doctor of Philosophy
Most stars form surrounded by a massive disk of dust and gas. As the star evolves, its interaction with the surrounding disk material has a significant impact on both the final evolutionary state of the star and the amount of material that is available in the disk to form planets, organic compounds, and, ultimately, life. Herbig Ae/Be stars (HAEBES) are recently formed intermediate mass (2-10 solar masses) pre--main sequence stars. Over 150 candidate HAEBES have been identified by various surveys. Although this number is large, it is relatively small compared to the number of identified classical T Tauri stars (CTTSs), the low mass (~1 solar mass) cousins of HAEBES. Although it is well established that both CTTSs and HAEBES are still evolving towards the main sequence, our understanding of how accretion and outflows operate around HAEBES compared to CTTSs is incomplete and there remains debate over the key launching mechanisms for the outflows in CTTSs. This is in large part due to the lack of comparisons of multi-wavelength mass flow diagnostics in large samples of HAEBES and CTTSs. In this thesis we attempt to address the gap in our knowledge of the driving mechanisms of accretion and outflows around HAEBES, and how they compare to those of CTTSs, by examining a wide variety of accretion and outflow diagnostics in the ultraviolet, optical, and near infrared with the goal of constraining the incidence of accretion and wind flows around these objects. A different incidence of accretion and outflow detections in HAEBES compared to CTTSs would indicate that the mechanisms governing the production of these flows in HAEBES differ from those in CTTSs and we can look to other key differences between these classes of stars to try to identify the mechanisms that control the flows. To accomplish this we analyze high resolution line profiles of a large number of spectral lines that are known to be good tracers of accretion and outflows around CTTSs. Our analysis reveals a significant difference between the occurrence of blue and red-shifted absorption features in HAEBES compared to CTTSs. This difference is largest for outflow signatures in the optical and is less significant in He I 10830, the near-IR diagnostic. We find that the incidence of red and blue-shifted absorption in HAEBES increases from the optical to the UV with intermediate rates being found in He I 10830. This suggests that hot (~100,000 K) mass flows are more common around HAEBES than cooler (~10,000 K) flows. We also find significant differences between the occurrence of red and blue--shifted absorption in HAe stars compared to HBe stars. In particular, our results support the idea of mangetospheric accretion occurring in HAe stars but provide more evidence for boundary layer accretion in HBe objects. In addition, we observe that the maximum red--shifted absorption velocities tracing infalling material in our sample are smaller fractions of the stellar escape velocity than is found for CTTSs. This is confirmed in both the optical diagnostics and at He I 10830. We suggest that this is a result of smaller magnetospheres mediating the accretion flows in HAEBES compared to CTTSs. We point out that smaller magnetospheres, and thus smaller infall velocities of material from the disk, will result in less energy being deposited onto the star which may in turn affect the launching of outflows. If winds from HAEBES are driven in part by accretion, which they appear to be in CTTSs, the smaller relative amount of energy deposited by the accretion flows onto HAEBES may result in less outflow activity, as observed. In particular, we observe a negligible fraction of objects that display simultaneous blue and red--shifted absorption signatures, a commonly observed trait in CTTSs. This supports the idea that accretion onto HAEBES is less efficient at driving outflows. Our results make it clear that the various mechanisms involved in controlling accretion and outflows in young stars do not, in general, operate in the same way around HAEBES and CTTSs.
Pre-main sequence stars; Herbig Ae/Be stars/ Classical T Tauri stars; accretion; stellar winds