HETG
Follow-on Science Instrument
Contract NAS8-01129
Monthly Status Report No. 007
September 2002
HETG Science Theme: "Cool" 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
“Cool” Stars Research Progress
Introduction to Stars
Stars are the main actors in creating variety in the universe. Their fusion “burning” of primordial H and He creates the elements we are made of and illuminates the universe.
In studying stars we consider two general categories: “Cool stars” which include stellar coronae, active binaries, and low-mass pre-main sequence stars; and “Hot” stars which consist of massive stars, associated winds and shocks, and stars in young star clusters.
Cool stars, those of spectral
type later than F0, are frequently X-ray sources with band-limited luminosities
(0.1-10 keV) on the order of 10^31 ergs/s. They are particularly active if
they are rapidly rotating, as the tidally-locked RS CVn (spotted stars)
binaries are. The activity is hypothesized to be due to a magnetic dynamo
resulting from differential rotation, as seen in the Sun. The dynamo is
presumed to be active only in cool stars because they have a convection zone
needed to drive the dynamo. Young, low-mass stars are also strong X-ray
sources, presumably because they have not yet lost their natal angular
momentum. Stars hotter that spectral type F are fully radiative with no
convection dynamo; for example, A-stars are well known to be X-ray dark.
Although the surface temperature of “Cool” stars is less than 10,000 degrees, the magnetic dynamo can create tempertures in the millions of degrees in the star’s cornoa. This coronal X-ray emission is a small fraction of the total stellar luminosity (10^-6 in the case of the Sun, 10^-3 in the case of the most active stars), but this energy is the primary radiative manifestation of the atmospheric heating by the magnetic dynamo; it represents the generation and rearrangement of magnetic fields from the star, and is an important source of interaction of the star with the surrounding medium.
With HETGS X-ray spectra, we can now clearly resolve the coronal X-ray emission into line and continuum components. As the comparison with an ASCA (CCD) spectrum at right demonstrates -- we can now “see the forest-floor between the trees”. From these measurements we can determine the plasma temperature structure, density, elemental abundances, and time variability. If we can detect any eclipses or rotational modulation, then we may also be able to constrain the geometry of the emitting structures.
Another
feature of an active corona is flaring activity. Injection of energy through
high-energy electrons causes heated plasma to expand while at the same time
being confined to loop structures by the local coronal magnetic fields. Such
loops are imaged on the Sun by TRACE and Yohkoh satellites and provide models
we can apply to other stars.
For more fantastic images and an overview of the Solar corona, see
http://vestige.lmsal.com/TRACE/POD/NAS2002v2.html and
http://isass1.solar.isas.ac.jp/ .
For more basic information on the Sun, Stars, and Stellar Evolution see
Lessons 3, 4, and 5 at http://cosmos.colorado.edu/astr1120/hypertext.html
"The dynamo is at the core of the activity problem. As argued in here, stellar activity, and therefore the stellar dynamo, is key to understanding life in the Universe and Earth's habitability. There is, however, no comprehensive model of solar and stellar magnetic activity.” K. Schijver (http://canopy.lmsal.com/schryver/ .)
Summary of “Cool Star” GTO Observations
and Activities
The stars in the HETGS GTO program are listed in the table below. There are five “cool” stars among them and some highlights of the observations are presented in the following pages. II Peg is a classical, highly active, single-lined RS CVn binary. AR Lac is a 1.98 day period RS CVn binary comprised of a K0 subgiant and a G dwarf. TW Hydra a pre-main-sequence star which is nearby, isolated, and a classical T Tauri object (artist’s impression at right): it's X-ray spectrum is unlike any of the other cools stars, possibly due to an accretion stream. A related object, TV Crt, has been accepted for AO-4. Finally, TY Pyx is a 3-day period near-totally eclipsing RS CVn system which showed eclipse modulation and a short flare.
|
Obs cycle |
obsid |
Type |
Target |
Binary? (period) |
Flare(s) seen? |
Comment |
|
1 |
1451 |
Cool |
II Peg |
7 d |
yes |
RS CVn binary; “two-ribbon” flare |
|
1 |
3,4 |
Hot |
Trapezium |
--- |
--- |
Many hot (and cool) stars |
|
1 |
5 |
Cool |
TW Hydra |
isolated |
yes |
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 |
|
2 |
599, 2420 |
Hot |
Iota Orionis |
--- |
--- |
Two O-stars; colliding winds? |
|
3 |
2525, 2526 |
Hot |
NGC 2362 |
--- |
--- |
Young (5-7 Myrs ) star cluster in the Galaxy; w/ massive stars |
|
4 |
3728 |
Cool |
TV Crit. |
Pair of binaries,1” sep. |
Not yet observed |
weak-lined T Tauri star; solar-mass “Vega-type'' (dust-disk) system; in TW Hydra assoc. |
HETG Observation of II Peg
|
II Peg (AO-1; Obsid 1451) is a classical, highly active, single-lined RS CVn binary with a period of 7 days. It is heavily spotted – the optical light curve amplitude has been as large as 0.5 magnitude. As shown at right, the HETGS spectrum clearly resolves a large number of emission lines peaking above the background continuum level. The brightest lines are from Ne and Oxygen and Fe lines are about 0.1 of Solar values. |
|
The HETGS observation showed a steady flux for about 25 ks, then an increase by about a factor of 3 in a classic Solar "two-ribbon flare" profile, then began a decay as the observation ended. This is shown in the plot at left by the behavior of the continuum level (dense black vertical lines) which is well fit by a flare model (green line.) The fluxes of discrete lines are shown as well.
A wide range of temperatures exist in II Peg’s corona with different temperatures giving rise to different patterns of emission lines. For a given set of measured lines and continuum it is possible to create a distribution of temperatures causing the emission. This “DEM” analysis has been carried for both the “quiet” pre-flare time and the flaring period of the observation. The plot below shows a broad range of temperatures between 3 million [LogT=6.5] and 100 million [8.0] degrees in the pre-flare state (green.) During the flare the high temperatures are increased with a peak around 45 million [7.65] degrees (grey).
Observations of AR Lac
AR Lac a 1.98 day period RS CVn binary comprised of a K0 subgiant and a G dwarf. Observations were made at 6 times to sample 3 phases twice each, covering quadratures and eclipses as shown in the count-rate plot at right.
There was much intrinsic variation, including one moderate flare (blue, peak at phase 0.47). No observations at repeated phases showed repeatable flux, though the quadrature observations (phase 0.2-0.3) largely overlap in flux. Eclipses were not detected. Both simultaneous and contemporaneous EUVE data were obtained in collaboration with J.Drake/CfA.
Using the “DEM” method mentioned previously (II Peg’s flare temperatures above), a DEM plot for AR Lac’s emission has been created and is shown at left. The solid line is the estmated DEM; the dashed lines give an estimate of the confidence range of the DEM. Features of the DEM are a low temperature (1.6 million [6.2] deg.s) and two high temperature (8 and 23 million [6.9,7.37] deg.s) peaks as well as a high-temp plateau region extending to above 100 million degrees [8.].
Ne and Fe variation in a set of Cool Stars
The Sun “is only one example of a large class of stars; a single example provides insufficient constraints to outline a path for dynamo theorists. In order to understand the solar dynamo, we need a population study: look at stars like the Sun, at young stars, old stars, binary stars, exotic stars, ... " K. Schijver (http://canopy.lmsal.com/schryver/ .)
The Chandra archive has a growing collection of high-resolution stellar X-ray spectra. Here are assembled HETG spectra of TW Hya and the active, late-type stars AB Dor (a rapidly rotating K dwarf, obsid 16), Capella (obsid 1318), HR 1099 (obsid 62538), AR Lac (6,9), TY Pyx, UX Ari (605), Lambda And (obsid 609), and II Peg in the spectral region encompassing the 13.56 A He-like Ne IX triplet and the 15.01 A Fe XVII line (see also Drake et al. 2001, Canizares et al. 2000, and Huenemoerder et al. 2001).
Spectra are shown as raw counts in the summed MEG +/-1st orders, in 0.005 A bins, and have been smoothed slightly. They are arranged from top to bottom roughly in order of the Ne IX 13.56 A to Fe XVII 15.01 A ratio, which is more sensitive to abundance than to temperature. Lines labeled are of Ne IX , Fe XVII-XVIII , and O VII.
“Cool” Stars Plans and Further Work
· AR Lac: finish paper with final revisions of co-author comments.
· 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 observations (AO-4) have not been scheduled. This star is a visual double, with 1 arcsec separation, and will present a spatial-spectral data analysis challenge. Continue exploring spatial-spectral analysis methods in preparation for these data.
· Improve the modeling of the density sensitive He-like triplets of Ne IX and Mg XI, which are blended with. Current results on these ratios should be considered preliminary and subject to revision.
· Compare and contrast more ensembles from many observations, both GO and GTO, like the Ne/Fe comparison plot shown above.
· A common feature of most observations to date is that flares are frequent. It may not be possible to detect uncompromised rotational modulation or eclipse signatures, which are crucial to density/volume estimates and magnetic loop models. Consider observations of less active, though fainter, stars for this purpose.