Late B-type stars and their candidate companions resolved with Chandra*
1
INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
2
European Southern Observatory, Casilla 19001, Santiago 19, Chile
3
Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
Corresponding author: B. Stelzer, stelzer@astropa.unipa.it
Received:
10
February
2003
Accepted:
13
June
2003
We present the first results from a series of Chandra
observations carried out with the aim to examine the origin of X-ray emission
in main-sequence late B-type stars.
X-ray detections of late-B and early A-type stars have remained a mystery
as none of the two major theories for stellar X-ray emission applies in this spectral range:
while O- and early B-type stars drive strong winds that are subject to instabilities,
late-type stars produce X-rays as a result of magnetic dynamo action.
Since any dynamo works only in the presence of a convective zone,
early-type stars are not
magnetically active. We use high spatial resolution X-ray observations
to enlighten the prevalent
speculation that previously unknown late-type or low-mass companion stars are
the sites of the X-ray
emission, instead of the B-type primaries. Here we present the results
for HD 1685, HD 113703, HD 123445, HD 133880, and HD 169978.
Adaptive optics observations have recently revealed at least one
faint object near each of these B-type stars (at separation of 
).
Four of the new infrared objects show infrared colors and magnitudes typical for low-mass
pre-main sequence stars, and are likely true companions to the ∼10–50 Myr old B-type
stars.
These multiple systems are now resolved for the first time in X-ray light.
We uncover that four of the new companions are X-ray
emitters, and the fifth one is likely to be a weak X-ray source below the detection limit.
Three of the B-type primaries are X-ray dark down to the detection limit of
erg/s. But we do detect X-ray emission from the position
of HD 1685 A and HD 169978 A.
The latter one indeed is a spectroscopic binary.
The characteristics of all X-ray sources are compatible with
those of typical young late-type stars:
hard X-ray spectrum (
keV) and high X-ray luminosity (
erg/s). Spectroscopic observations in the infrared
will solve the question whether the one remaining
X-ray detected B-star in our sample, HD 1685 A,
also has an even closer companion or whether this is an intrinsic X-ray emitter.
Key words: X-rays: stars / stars: early-type, late-type, coronae, activity
© ESO, 2003