Issue |
A&A
Volume 509, January 2010
|
|
---|---|---|
Article Number | L7 | |
Number of page(s) | 5 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/200913704 | |
Published online | 19 January 2010 |
Letter to the Editor
The nature of the recent extreme outburst of the Herbig Be/FU Orionis binary Z Canis Majoris *,**
1
European Southern Observatory, Alonso de Cordova 3107, Casilla
19001, Santiago 19, Chile e-mail: tszeifer@eso.org
2
Astronomisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam,
Germany
3
European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching,
Germany
4
INAF-Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134
Palermo, Italy
5
Department of Theoretical Physics and Astrophysics, Masaryk University,
Brno, Czech Republic
Received:
19
November
2009
Accepted:
17
December
2009
Context. Z CMa is a binary system which consists of two young stars: a Herbig AeBe component Z CMa NW embedded in a dust cocoon and a less massive component Z CMa SE, which is classified as a FU Orionis type star. Associated to the binary system is a giant parsec-size jet. Past spectropolarimetric observations showed that the position angle of the linear optical polarization is perpendicular to the jet axis, indicating that the visual light escapes the cocoon via cavities aligned with the jet axis and is then scattered back into the line of sight of the observer. Recently the system showed the largest outburst reported during the almost 90 years of available observations.
Aims. We present new spectrophotometric and spectropolarimetric data obtained in 2008 during the recent outburst phase.
Methods. The data obtained in the visual spectral range at medium spectral resolution were used to study the geometry of the system from the linear polarization spectra as well as its magnetic field from the circular polarization spectra.
Results. During the recent outburst we detected that the Z CMa system is
polarized by 2.6% in the continuum and emission line spectrum, with a
position angle still perpendicular to the jet. From the high level of
polarization we
concluded that the outburst is associated with the dust-embedded Herbig
AeBe NW component. The deep absorption components of the Balmer lines in
the velocity frame which extend from zero velocity and reach a wind
velocity of ~700 km s-1, together with the absence of a
red-shifted broad emission at similar velocities, indicate a bi-polar
wind.
We did not detect a significant mean longitudinal magnetic field during
the outburst, but in the data obtained in 2004 we detected the
possible presence of a rather strong magnetic field of the order of
~1 kG.
However, we critically review the applied method of magnetic field
measurements in the presence of a strong stellar wind.
The main result of our studies is that the bolometric luminosity of
Z CMa remained surprisingly constant during the recent outburst.
We conclude that either the geometry of the cavity through which the
light escapes from the cocoon has opened a new path or that the screen
of dust, which reflects the light toward the observer became more
efficient, causing the observed increase of the visual brightness by
about 25.
Key words: stars: individual: Z Canis Majoris / stars: pre-main sequence / stars: winds, outflows / stars: magnetic field / stars: variables: general / binaries: close
Based on observations made with ESO Telescopes at the Paranal Observatories under programme IDs: 074.C-0442, 081.C-0410, and 282.C-5041.
Appendix A, Tables 1 and 2, and Fig. 6 are only available at http://www.aanda.org
© ESO, 2010
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