1. Chandra Grating Spectroscopy of the Hot Star  Crucis and the Discovery of a Possible Pre-Main Sequence Companion
Michael A. Kuhn, David H. Cohen, Eric L. N. Jensen (Swarthmore College), & Marc Gagné (West Chester University of Pennsylvania)
Summary
b Crucis and Its Properties
The Chandra Data
We obtained a Chandra grating observation of the star b Cru to investigate x-ray generation by early B stars. This observation led to the fortuitous discovery of a new x-ray source at 4 arc seconds from b Cru. From the observation, we argue that b Cru’s x-ray emission lines originate in a stellar wind. We also find evidence suggesting that the new x-ray source is a pre-main sequence companion to b Cru.
We present a 74,000 s Chandra grating observation. The ACIS CCDs are shown at the right, along with the positions of the six PMS stars detected by Park & Finley (1996). Note that the dispersed MEG spectra are visible ( ) as is the “zeroth order” spectrum ( ).
b Cru (Becrux, Mimosa, HD ) is a member of the Lower Centaurus Crux subgroup (LCC) of the Sco-Cen OB Association.
With L = 3.4 X 104 Lsun and a distance of only 108 pc, b Cru is also the 19th brightest star in the sky and as one of the four bright stars in the Southern Cross, it appears on five national flags and numerous other flags.
At the position of the zeroth order spectrum are two sources, separated by 4.0 arc seconds! The slightly brighter one to the northwest is  Cru.
Scientific Questions to Address
Spectral Type……..………….B0.5 IV
Mass…………..………………16 Msun
Age…………..……….….8 to 11 Myr
Teff………..…..…… / K
R……………….………8.4 +/- 0.6 Rsun
L…………………..……3.4 X 104 Lsun
vsini…………………………..35 km/s
O star x-ray emission is from instability-generated shock-heating in their strong, radiation-driven stellar winds. How does this wind-shock process operate in less luminous B stars with much weaker winds?
b Cru is a beta Cephei variable – with several closely spaced non-radial pulsation modes (P ~4 hr). How does this photospheric variability affect the x-ray production?
With an age of ~11 Myr, the LCC subgroup of the Sco-Cen OB association harbors many low-mass pre-main-sequence stars that are strong x-ray sources, including a small group of stars within 0.5 degrees of b Cru, discovered by ROSAT (Park & Finley, 1996). How does the cluster membership affect the x-ray properties of the  Cru system?
Red: E<0.5keV - Green: 0.5-1keV - Blue: >1keV
This possible companion has never been seen in the optical because of the extreme brightness (mV=1.25) of  Cru.
Brazil
Luckily, the dispersion direction (marked by parallel lines in figure at left) for the roll angle of our observation was nearly perpendicular to the position angle of the companion. On the right is an emission line seen in the two sources (image). As the inset, which shows a cut perpendicular to the dispersion direction, confirms, the spectral features are well-separated.
Binarity: Spectroscopic companion (B2 V), P=5 yr; also a claimed 45” visual companion, which was later shown to not be physically associated.
Variability: beta Cephei variable, with pulsation periods of hr, hr, hr
secondary
Australia
Christmas Island
primary
The Chandra grating spectrum
X-ray Properties of the Newly-Discovered Companion
Implications of the small, but resolved, line widths
Can the small line widths (~200 km/s) be understood in terms of wind shocks if the theoretical wind terminal velocity of ~2000 km/s is correct?
No: we fit line profiles synthesized assuming a smooth, spherical, beta-velocity-law wind, with emissivity scaling as the square of the wind density above some minimum onset radius, Rmin, which we took to be a free parameter. To explain the narrow lines in a fast wind, Rmin must be <1.01 Rstar - so all the x-rays originate at the base of the wind. This inconsistent with the standard wind-shock scenario.
But, if the wind terminal velocity is only 420 km/s, as the UV line profiles indicate, then the observed half-widths of ~200 km/s are completely consistent with the wind-shock scenario. When we fit the wind profile model with this lower terminal velocity, we find Rmin values for several lines that range between 1.3 and 1.5 Rstar.
The spectrum of the companion is much harder than that of the primary. On the right we show the zeroth order spectrum (with that of the primary overplotted). The best-fit 2-T APEC model gives temperatures of 9 and 23 MK. The signal in the first-order MEG spectrum is not strong enough to do a significant amount of analysis but is consistent with the thermal model used.
v∞ = 420 km s-1
Primary
v∞ = 2000 km s-1
Secondary
The wind profile model is unable to produce a good fit with v∞ = 2000 km/s and Rmin = 1.3 Rstar.
The spectrum is very soft compared to coronal sources and even softer than most O star spectra – we derived temperatures of roughly 1 and 3 MK, fitting a 2-T thermal (APEC) model to the MEG+/-1 spectrum (above) and the low-resolution zeroth order spectrum (third column, at right).
The companion shows variability, which appears to be stochastic, and which is apparent in both the hard and soft bands. The light curve shown at the right is inconsistent with a constant source with >5 sigma significance. While not obviously flare-like in its morphology, it is similar to the x-ray light curves of active coronal sources.
z Pup (O4)
b Cru
Thermal Broadening
Best Fit
What fraction of the wind is shock-heated? We can integrate all the emission measure above Rmin assuming a smooth, spherical wind:
If we assume a velocity law and an Rmin value (or take the value from the line profile fits), the emission measure is determined by the mass-loss rate. In order to explain the observed emission measure under these assumptions, Mdot must be at least 10-9 Msun yr-1. A large fraction of the wind above Rmin = 1.3 to 1.5 Rstar must be hot.
We can measure the f/i ratio in Ne IX for the companion, and we find that it is in the low density limit (f/i = 2.2 +/- 1.1) – there is no weakening of the forbidden line due to collisional excitation. This is consistent with the f/i ratios of active low-mass coronal sources and wTTS stars. Some cTTS stars show an altered f/i ratio.
Ne IX f,i,r Complex
resonance
intercombination
forbidden
Time Variability of the X-ray Emission
The most unusual characteristic of the spectrum is the widths of the emission lines – they are quite narrow, but resolved, with a typical half-width of only 200 km/s (widths of all lines summarized on the left). On the right, a model of oxygen Lya with only thermal broadening (half-width ~50 km/s) is also plotted along with the best fit Gaussian; the model with only thermal broadening is ruled out at the 5 sigma level. This line is clearly much narrower than the corresponding line in z Pup (shown in inset).
Conclusions
What does it mean?
Observed C VI blue edge velocity is -420 km/s.
95%
The primary – b Cru (B 0.5 IV) – has x-ray properties that are consistent with the wind-shock mechanism operating in most other early-type stars, but only if the terminal velocity of the wind is quite small. A large fraction of the wind above 1.3 to 1.5 Rstar must be shock-heated, and remain hot, which is consistent with the low wind density (and large cooling lengths).
The x-ray properties of the companion are completely consistent with those of typical low-mass PMS stars or active ZAMS stars – relatively hard thermal spectrum with variability. The x-ray luminosity of is consistent with those of the ROSAT-discovered PMS stars in the vicinity of b Cru. Optical photometry – if not spectroscopy – of the companion would be very useful for determining whether the companion is a low-mass PMS star. An optical measurement or a short follow-up x-ray observation would also be valuable in confirming that the two objects are comoving. If the PMS status of the companion can be confirmed, then the b Cru system would join the ranks of Lindroos binaries – B type stars on the main sequence or post-main-sequence, with low-mass PMS binary companions.
The expected wind properties, based on CAK wind theory, are M-dot ~ 10-8 Msun yr-1, vinf ~ 2000 km/s: This would lead to a typical x-ray emission line HWHM of about 1000 km/s, which is not observed. However, if the terminal velocity of the actual wind is much lower, as the IUE data (right) indicate, then wind-shock heating is a plausible model.
99%
There is no evidence of stochastic variability in the data – they are statistically consistent with a constant source (binned light curve on the left, above). However, there is evidence for periodic variability on the known optical period in the hard x-rays (E > 1keV) (see upper right for periodogram; three optical periods are indicated with vertical lines; the horizontal lines show the 95% and 99% significance levels). At the right we show a fit of a sinusoidal model to the binned hard light curve. There is no corresponding variability detected in the soft bandpass (despite the signal-to-noise ratio being higher there).
Presented at the 207th AAS meeting, Seattle WA, Jan. 2007
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