Trends in the Chandra X-Ray Spectra of O and Early B Stars

June 24, 2008

Spectra of Stars

• We know that spectra result from physical processes within stars.

• They often reflect physical properties of the stars that created them.

ex. Optical Spectra

X-Ray Spectra of O and Early B Stars

• The spectra on the left are a representative sampling of O and early B star spectra, with earlier spectral types at top.

• The spectra were taken on the Chandra X-Ray telescope using the high energy transmission grating (HETG) on the advanced camera for imaging spectroscopy (ACIS).

• Nolan Walborn suggested in a 2006 paper that these spectra may reveal a temperature trend similar to that observed for optical spectra.

• I will be looking for evidence that tends to confirm or disprove this hypothesis.

X-Ray Production in Hot Stars

• X-Ray emission arises in the stellar winds of these stars.

• The stellar winds are radiatively driven.

• The line-driven stellar winds are inherently unstable.

• Shocks in the wind arise from the instability and produce x-rays.

X-Ray Spectra and Temperature

• Line strength ratios• Differential Emission Measure

dEM= nenH dV

In a plasma with a single well-defined temperature, or

D(T)= ne(T)nH(T)(dV/d logT)

in a plasma with a temperature distribution.

Stars and ParametersHD93129AB

O2 If* + (2x) O3.5 V((f+))Teff = 50000 K

R= 20 Rsun

M=120 Msun

vinf = 3200 km/sdM/dt = 1.8 * 10-5 Msun/yr

Xi Persei

O7.5 III(n)((f))Teff = 36000 K

R= 12 Rsun

M=34 Msun

vinf = 2450 km/sdM/dt = 2.0 * 10-6 Msun/yr

Delta Orionis

O9.5 II + B0.5 IIITeff = 33000 K

R= 17 Rsun

M=23 Msun

vinf = 2000 km/sdM/dt = 1.07 * 10-6 Msun/yr

Silicon XIV/Silicon XIII Ratios

HD93129AB

0.05 ± 0.02

Xi Persei

0.05 ± 0.02

Delta Orionis

0.21 ± 0.06

Magnesium XIV/Magnesium XIII Ratios

Xi Persei

0.12 ± 0.02

HD93129AB

0.27 ± 0.04

Delta Orionis

0.24 ± 0.03

Conclusions (for now)

• Line ratios of stars in our sample seem to have little to do with effective temperature.

• This tends to disprove Walborn’s suggestion that the trend in the spectrum is temperature-related.

• However, there does appear to be some slight tendency for individual lines to become broader and more blueshifted in the hotter stars.

Some Directions for Research

• Continue to measure line ratios in more stars.

• Possibly explore measuring line ratios for other elements.

• Look into measuring emission measures for our sample stars as another probe of temperature.