HETG
Follow-on Science Instrument
Contract NAS8-01129
Monthly Status Report Numbers 020 & 021
October & November 2003
Science Theme: "Hot" and "Cool" Stars, Part II
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” and “Cool” Stars Progress, Part II (October’02 – November’03)
Summary of ”Cool” Stars Observations and Activities
In the past year on the “Cool” star front the analysis of AR Lac was completed and a paper published on it in the ApJ. Also the Cycle 4 GTO observation of TV Crit was carried out and the data have been initially analyzed. Both of these are detailed below.
Table of HETG GTO “Cool” star observations.
|
Obs cycle |
obsid |
Type |
Target |
Binary? (period) |
Flare(s) seen? |
Age (M yrs) |
Comment |
|
1 |
1451 |
Cool |
II Peg |
7 d |
yes |
--- |
RS CVn binary; “two-ribbon” flare |
|
1 |
5 |
Cool |
TW Hydra |
isolated |
yes |
~ 5 |
extremely cool, very low iron abundance, high neon abundance. |
|
1 |
6-11 |
Cool |
AR Lac. |
1.98 d |
yes |
--- |
RS CVn binary; |
|
2 |
601 |
Cool |
TY Pyx. |
3 d |
yes |
--- |
near-totally eclipsing RS CVn |
|
4 |
3728 |
Cool |
TV Crit. |
Pair of binaries,0.8” sep. |
yes |
~ % |
weak-lined T Tauri star; solar-mass “Vega-type'' (dust-disk) system; in TW Hydra assoc. |
AR Lac paper published --- Ap.J. 595 (2003) 1131-1147, and at astro-ph/0306380.
We observed the coronally active eclipsing binary AR Lac with the HETGS for a total of 97 ks, spaced over five orbits Significant variability in both X-ray and EUV fluxes was observed, dominated by at least one X-ray flare and one EUV flare (see Sept. 02 report.)
Based on line
fluxes obtained from total integrated spectra, Figure at right, we have modeled the emission measure and abundance
distributions. A strong maximum was found in the differential emission
measure, characterized by two apparent peaks near 10 MK and 30 MK, together
with a weak but significant cooler maximum just below 2 MK, and a moderately
strong hot tail from 40 MK to almost 200 MK.
The resulting model spectra generally agree very well with the observed spectra, see the enlarged region from 12.3 to 13.6 A in the Figure at right: the black lines are the MEG & HEG data and the red lines are the model. There are, however, some significant discrepancies, especially among the many Fe-L-lines.
Both the emission measure and abundance distributions are qualitatively similar to previous observations indicating some long-term stability in the overall coronal structure.
Our observations include different phases of the binary system, specifically at quadratures and conjunc-tions. Analysis of spectral line widths at these two phases of the system, Figure at left, can be interpreted in terms of Doppler broadening and suggests that both binary stellar components are active.
The data before and during the observed flare were analyzed separately to understand the flaring phenomena. The amount of change or modulation in fluxes from detected ion species is shown in the Figure at left plotted as a function of the temperature of peak emission for the ion. The X-ray flux modulation was largest at high temperature - at and above 10 MK. This indicates a mechanism of flare heating of coronal plasma rather than simply changes in emitting volume or overall electron/ion densities.
TV Crit initial results
During the past year out Cycle 4 target TV Crt was observed and initial analyses have been carried out. Chandra’s zeroth-order image, Figure below left, clearly resolves the principle binary components: A (red circle) and B (green circle), separated by 0.8 arc seconds. (Sub-pixel event position corrections were applied as described in Li et al., ApJ, 590, 586.) The plot in the Figure at right shows the count rate over the observation for the four circular regions indicated: total counts (black), central component (red), component B (green) and a southern region (blue.) A clear flare is seen in the light curve of the central source.
The HETG
spectrum from the combined components was formed and is dominated by highly
ionized Ne, O, and Fe at wavelengths longward of 12 Å. Weaker lines of
Mg, Si, and Ar are present shortward of this value.
Line ratios R = f/i and G = (f + i)/r are measured for the Ne IX triplet, and are plotted in the Figure at left for TV Crt (lower contours) and for the classical T Tauri star TW Hya (upper contours). The fish-net-like grid overlaid on the plot (with “axes” log n vs. log T) illustrates how the ratio R serves as a diagnostic of the density of the gas at the temperature indicated by G. Taken at face value the model densities implied for the post-accretion-phase TV Crt are almost a factor of 10 below those of still-accreting TW Hya.
The red (central), green (middle), and blue (outer) contours represent 99%, 90%, and 68% confidence levels in the measured values of R and G.
One goal of the observation was to look for differences in the spectra of the two components. As an example of the data available, the Figure at right shows the MEG minus and plus first-order dispersed events as a function of wavelength and cross-dispersion location, dy. The curve in green at right shows the projection of the zeroth-order image and the centers of the two components are indicated with horizontal lines. Analysis of differences between the two components in on-going.
Summary of “Hot” Stars Observations and Activities
On the “Hot” stars front, another paper on the Trapezium hot stars was published and a DEM survey of hot stars, including the GTO targets iota Ori and tau Cma, is in draft form – these topics are detailed below. A further observation of hot massive stars in the Orion Trapezium area has been added to our cycle 5 GTO program.
Table of HETG GTO and related “Hot” star observations
|
Obs cycle |
obsid |
Type |
Target |
Line shape |
T range (MK) |
Age (M yrs) |
Comment |
|
1, 3 |
3,4, 2567, 2568 |
Hot |
Trapezium |
--- |
--- |
|
Many hot (and cool) stars. [2567,’8 are now-public GO HETG observations, PI=Gagne.] |
|
“ |
“ |
Hot |
Theta-1 Ori C In the Trapezium HD 37022 |
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. |
|
5 |
4473, 4474 |
Hot |
Orion Nebula Cluster, Theta-1 Ori |
--- |
--- |
--- |
To be observed near end of 2004. |
Trapezium
hot stars progress --- ApJ 595 (2003) 365-383, and astro-ph/0306008
Detailed analysis has continued on the hot stars in the Orion Trapezium, in particular components C, E, and A shown in the image at right.
The brightest component, theta-1 Ori C, has been extensively modeled and shows generally narrow lines agreeing with model fits, Figure below right, with the exception of two low-energy lines, O VIII and Fe XVII, which show clear broadening over the model response, Figure below left.
From top to bottom the Figures here show the extracted spectra from the three brightest Theta-1 Ori components: C, E, and A respectively.
These three show
similarlly narrow lines suggesting they all involve magnetic wind confinement.
DEM survey of hot stars
In order to extract information from the spectra of hot stars (and other objects as well), we have developed a technique to measure the effective emmission measure distribution of the object using multi-component fits to the spectra (Wojdowski et al., 2004, in prep.). Each component is the spectrum due to a single ion, e.g. O VII or Fe XVII.
The result of these fits is shown by the emission measure vs temperature plot, Figure above right, where each ion species measured provides a generally diamond-shaped plot symbol indicating in width the temperature range covered by the ion and in height the averaged emission measure for the ion. The Figure at right above shows the MEG spectral data for theta-1 Ori C in black and the resulting multi-ion-components model spectrum in red.
This technique has the advantage of using all of the data and being less sensitive to line blend effects that a line-by-line measurement approach. Work on this technique and its application to model interpretation and determination for hot stars and other systems is on-going.
“Cool” Stars Plans and Further Work
· TY Pyx: Finish detailed analyses
o Measure line strengths
o fit the emission measure distribution and abundances
o extract light curves in continuua and lines
o examine differences in spectra for different count rates
· TV Crt: This star is a visual double, with 1 arcsec separation, and presents a spatial-spectral data analysis challenge – work to see if differences in the two sources can be discerned.
· Compare and contrast more ensembles from many observations, both GO and GTO, like the Ne/Fe comparison plot shown in the September 2002 report (and included in the Jan. 2003 Chandra Newsletter.)
· Design code and software for feature fitting database including lines, continua, absorption lines, edges, etc..
“Hot” Stars Plans and Further Work
· Focus on the low mass population of young T-Tauri stars when extracting more spectra from the Trapezium observations including the Gagne GO observations.
· Finish DEM survey of “Hot stars” including iota Ori and tau Cma.
· For Cycle 6 continue with ~ 100 ks exposures around the Trapezium.