HETG
Follow-on Science Instrument
Contract NAS8-01129
Monthly Status Report No. 008
October 2002
HETG Science Theme: "Hot" Stars
Prepared in accordance with DR 972MA-002
DPD #972
Prepared for
National Aeronautics and Space Administration
Marshall Space Flight Center, Alabama 35812
Center for Space
Research; Massachusetts Institute of Technology; Cambridge, MA 02139
"Hot" Stars Research Progress
Introduction to “Hot” Stars
Last month we desribed “Cool stars” which include stellar coronae, active binaries, and low-mass pre-main sequence stars all of which produce X-rays through coronal emission powered by the stars magnetic dynamo. In contrast “Hot stars”, consisting of early type stars (O,B), are thought not to have the convective dynamo action but instead X-rays are produced by shock instabilities in a radiatively driven wind. Some of the detailed mechanisms of this process are not yet understood. Specifically emission mechanisms in young massive stars are notoriously under-studied.
The HETGS high-resolution spectra provide new and more powerful possibilities to diagnose the emission from the hot star plasmas and understand the underlying mechanisms. In addition because young stars are generally found in dense stellar cluster cores, the spatial resolving power of Chandra is essential. These diagnostics include: line identifications, line ratios with their dependence on physical parameters such as temperature and density, line shapes and shifts, and global fitting of plasma models.
In particular the canonical wind model produces blue-shifted, broadened (P-Cygni) profile lines with emission at moderate X-ray temperatures, say up to 10 million degrees K. In some stars even higher temperatures are seen and one possible mechanism is magnetically confined wind shocks. If the magnetic field is high enough and the wind mass loss rate low enough then the outflow gets dominated by magnetic field effects and dense high-temperature regions can form. The diagram below (from http://www.bartol.udel.edu/~owocki/ ) shows the confinement effect on hot plasma as the magnetic confinement parameter increases from left to right. Note the increase of dense (red) regions.
Summary of “Hot Star” GTO Observations and Activities
Three HETGS “Hot star” targets have been observed to date as listed in the table below. The Orion Trapezium contains a huge number of X-ray point sources (e.g., see next page) and in particular the X-ray-brightest member, Theta-1 Ori C, is young and massive with very high temperature emission. The other two objects, iota Ori and tau Cma, have spectra more similar to the prototype wind shock object zeta Puppis.
|
Obs cycle |
obsid |
Type |
Target |
Line shape |
T range (MK) |
Age (M yrs) |
Comment |
|
1 |
3,4 |
Hot |
Trapezium |
--- |
--- |
|
Many hot (and cool) stars |
|
“ |
“ |
Hot |
Theta-1 Ori C In the Trapezium |
Narrow, Symmetric |
8-74 |
0.3 |
Magnetically confined wind shocks?! 25 solar mass star. |
|
2 |
599, 2420
|
Hot |
Iota Orionis |
Broad, Symmetric, Most flat-topped |
1-10 |
< 12 |
Two O-stars; colliding winds? |
|
3 |
2525, 2526 |
Hot |
NGC 2362, Tau Cma
|
Broad, Symmetric, Flat-topped |
3-22 |
> 3 |
Young (3-7 Myrs ) star cluster in the Galaxy; w/ massive stars. |
The Orion Trapezium – A Stellar Nursery (Schulz et al., ApJ 549, 441S, 2001)
The Orion Trapezium Cluster, only a few hundred thousand years old, offers a prime view into a stellar nursery. Its X-ray sources detected by Chandra include several externally illuminated protoplanetary disks ("proplyds") and several very massive stars, which burn so fast that they will die before the low mass stars even fully mature.
The time variability of many of the sources in the field is shown by the two Chandra views above taken a few weeks apart as some objects brighten and fade between the exposures.
HETGS Observation of the Orion Trapezium
(Schulz et al., ApJ 545L, 135S, 2000; Schulz et al. 2003, in preparation)
The HETGS observation of the Trapezium shows spectra (the linear streaks) from the bright objects in the field. The brightest source in X-rays is the source Theta-1 Orionis C whose spectra are indicated in blue above and is the dominant central source in the blowup of the central region at right.
Spectra from ‘C and the other bright sources in the field were extracted in the standard way sometimes using a narrowed cross-dispersion selection range and correcting the fluxes appropriate for the width reduction. Each spectra was also cleaned of contributions from interfering field sources. Some crossing of the spectra is seen and ACIS energy discrimination was able to eliminate spectral contamination.
Analysis of Theta-1 Ori C
High resolution spectra from Theta-1 Ori C were obtained at two phases of its approximately 15 day (assumed to be) rotational period. Using the measured lines in the spectra emission measure distributions (“DEM”) were created using the same algorithms employed for “Cool star” analysis.
The DEM results for the two phases are shown at right by the red and green curves – the curves for a given color define the upper and lower confidence range for the emission measure. These curves show a relatively phase-independent emission peak at just below 10 million degrees K (peaking ~6.95 in the plot) and more dramatic variations between phases in the 25 to 60 million degree temperature range (7.4 to 7.8 in LogT.)
Combining this phase modulation with the narrowness of the emission lines observed is strongly indicative of a magnetic confinement mechanism for the high-temperature emission.
Contrasting Theta-1 Ori C with tau Cma and iota Ori
The counts spectra for theta-1 Ori C, tau Cma and iota Ori, are shown respectively above. The temperature of the sources decreases from left to right. Even at a glance it is clear that theta-1 Ori C is a “different beast” from the two other “Hot stars”: it has proportionally far more short-wavelength continuum and its lines are narrow. In addition Theta-1 Ori A and E, the next brightest X-ray emitters in the Trapezium, also show similar “magnetic” characteristics in their HETGS spectra. This raises the question if young massive stars enter the main sequence with significant magnetic fields at moderate outflow rates.
"Hot" Stars Plans and Further Work
· Finish current Trapezium paper (paper “3”.)
· Extract more spectra of massive stars from the Trapezium observations including a newly available AO-2 observation of 100 ks more than doubling the current exposure.
· Finish DEM survey of “Hot stars” including iota Ori and tau Cma.
·
Select
and propose the next best candidate for magnetic confinement in “Hot
stars” for AO-5.