ChandraChandra Emission Line Diagnostics of Emission Line Diagnostics of Sco ScoGeneviève de Messières (Swarthmore College ‘04), Carolin Cardamone (Wellesley College ‘02), David H. Cohen (Swarthmore College),

Joseph MacFarlane (Prism Computational Sciences), Stanley Owocki & Asif ud-Doula (Bartol Research Institute, University of Delaware)

Presented at the American Astronomical Society winter meeting - Washington, DC - January 6-10, 2002

ROSAT (1993) spectrum of Sco

Chandra (2000) MEG spectrum (85 ksec)

The quality of astrophysical X-ray spectra has improved dramatically in the last decade: The high-resolution of the Chandra HETGS allows us to study the profiles of spectral lines in Sco to determine the velocity structure

of the X-ray-emitting material, and line-intensity ratios to determine its height above the photosphere.

The processes by which hot stars produce X-rays are not yet fully understood

- Cooler stars like the Sun generate X-rays through magnetic confinement and heating of a corona. However, hot stars are generally thought to lack the convective envelopes and magnetic fields assumed to be necessary for coronal X-ray production. - The leading theory for hot stars X-ray production is shock-heating in radiatively-driven winds. The line-force instability (Owocki, Castor, Rybicki 1988; Feldmeier 1995) is a natural mechanism for generating shocks in a radiation-driven wind. - A third, hybrid, mechanism is the magnetically confined wind shock (MCWS) model (Babel and Montmerle 1997). A star with a magnetic field and a substantial radiation-driven stellar wind can develop a disk of colliding plasma around its magnetic equator that would generate shock-heated plasma and associated X-rays. Asif ud-Doula is discussing dynamical modeling of the MCWS mechanism at these proceedings (Poster 135.07).

Line Widths Provide Velocity InformationLine Widths Provide Velocity Information

(Left) The Ne X Ly- line from the spectrum of Sco in comparison to lines from Capella (which

is a cool coronal source) and Puppis (which has a strong stellar wind with embedded shocks).

Roban Kramer has studied the broadened line profiles of stars with stronger winds (Poster #135.13).

There is only modest broadening of Sco’s spectral lines. A fit of a delta-function line-profile model of the Ne X line shows that the

actual width exceeds the instrumental broadening, but only by several 100 km s-1.

Our results: Lines in Sco are not as narrow as those in coronal sources, but they are only slightly broadened. Typical velocities are 200 km s-1, while the observed wind terminal velocity for Sco is at least 1500 km s-1. Furthermore,

the lines are slightly redshifted (mean value ~50 km s-1 for ~20 lines).

Line Ratios Provide Information About LocationLine Ratios Provide Information About Location

Mg XI

Si XIII

We can resolve three closely spaced lines in each helium-like triplet: the resonance (R), intercombination (I), and forbidden (F) lines. The ratio of the F to the I line is a diagnostic of the strength of the ultraviolet field, and therefore distance from

the star (because the strength of the UV field is directly related to radius through the dilution factor).

In a strong UV field, electrons can be

excited out of the metastable upper

level of the forbidden line (3S 3P) before they spontaneously

de-excite, weakening the F line in favor of

the I line.

Our results: We found that the hot plasma is located between 1.5 and 5 stellar radii from Sco, with the majority located at

about 2-3 radii. The Si XIII data suggest that the hottest material is nearest the star, but there are uncertainties in the

photospheric EUV flux that may affect this result.

Ion F/I Error Range of radii (r/R)

oxygen VII 0 0.01 (est.) < 5

neon IX 0.0845 0.0214 2.2 - 3

magnesium XI 0.3057 0.0654 1.8 - 2.5

silicon XIII 2.1623 0.286 1.1 - 1.5

WINDRT simulations (MacFarlane et al 1993; MacFarlane, Cohen, & Wang 1994) model the UV field strength necessary to cause the observed F/I ratios.

Observed range for tau Sco

WINDRT simulations of neon IX

DiscussionDiscussion - The physical picture that emerges from these diagnostics suggests very hot plasma that is a moderate distance from the star (1 to 2 R* above the photosphere) and that is not moving very fast. Some of it, in fact, appears to be falling toward the star.

- This is inconsistent with both the coronal and line-force instability models. (The material is too far from the star to be explained by a coronal model, but is moving much more slowly than a stellar wind driven by line-force instability.)

- A possible alternative: The magnetically confined wind shock model model naturally explains how wind plasma could be shock-heated at a significant distance from the star, while remaining relatively stationary.

The MCWS model: A large-scale magnetic field can channel ionized wind material toward the magnetic equator, where it collides with material from the opposite hemisphere, leading to a strong standing shock and generating X-rays. (See ud-

Doula & Owocki, poster 135.07.)

Y- Velocity

-1000 vy (km/s) 1000

Density

Sco is an unusually young star (Kilian 1994), and it could retain a primordial magnetic field from its formation, thus it is a potential candidate for the MCWS mechanism. A polar magnetic field strength of just a few 10’s of G in this star could lead to significant magnetic effects (* ~ 1).

Preliminary analysis indicates a significant quantity of plasma with temperature in excess of 107 K, hotter than the X-ray emitting plasma seen in most OB stars, and potentially explained in the context of strong magentic wind shocks (much like 1 Ori C).

Line centroid redshifts might be explained if the shock-heated plasma arises from the interaction of density condensations in the wind, falling back toward the star, as has been modeled in this star by Howk et al. (2000).

The periodic infall of wind condensations is an inherent property of the MCWS model (‘magnetospheric emptying’). See simulations by ud-Doula and Owocki (Poster 135.07).

laboratory rest-wavelength

ASCA (1997) spectrum

HWHM of selected lines for 7 stars

-500

0

500

1000

1500

1 2 3 4 5 6 7

Capella Sco ζ Pup ζ Ori θ Ori δ OriAB Dor

Ne X (12.13), Fe XVII (15.01), O VIII (18.97)

Cool Stars(Coronal)

Hot Stars(Wind)

Ne IX

O VII

Scorpii

B0 V, mv= 2.8

D = 132 pc

Teff = 31,400 K

Vsini = 20 km s-1

M ≈ 10-9 to few X 10-8 Msolar yr-1

v∞ = 1500 km s-1

Age ≈ 1 Myr

Redshifted as well as blueshifted UV wind absorption from Copernicus

http://hven.swarthmore.edu/~cohen/group/AAS2002_GdM/