Reviews in Modern Astronomy: Variabilities in Stars and Galaxies

278 Pages·1992·16.3 MB·English

by  B. Wolf (auth.)

Reviews in Modem Astronomy
5
Gerhard Klare (Ed.)
Reviews in
Modem Astronomy 5
Variabilities in Stars and Galaxies
With 122 Figures
Springer-Verlag
Berlin Heidelberg New York
London Paris Tokyo
Hong Kong Barcelona
Budapest
Edited on behalf of the Astronomische Gesellschaft by
Dr. Gerhard Klare
Landessternwarte, Konigstuhl,
0-6900 Heidelberg 1, Fed. Rep. of Germany
ISBN-13: 978-3-642-77545-1 e-ISBN-13: 978-3-642-77543-7
DOl: 10.1 007/978-3-642-77543-7
This work is subject to copyright. All rights are reserved, whether the whole or part of the material is
concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcast
ing, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this
publication or parts thereof is permitted only under the provisions of the German Copyright Law of
September 9, 1965, in its current version, and permission for use must always be obtained from Sprin
ger-Verlag. Violations are liable for prosecution under the German Copyright Law.
© Springer-Verlag Berlin Heidelberg 1992
Softcover reprint of the hardcover 1s t edition 1992
The use of registered names trademarks, etc. in this publication does not imply even in the absence of a
specific statement, that such names are exempt from the relevant protective laws and regulations and
therefore free for general use.
Typesetting: Camera ready by author
55/3140-543210 - Printed on acid-free paper
Variabilities in Stars
and Galaxies
Preface
The International Scientific Spring Meeting of the Astronomische Ge
sellschaft (AG) held at Bamberg in April 1991 was devoted to "Vari
abilities in Stars and Galaxies".
Time-dependent phenomena are observed in a wide range of as
tronomical objects. They are caused by different physical mechanisms
(for example by pulsation, by accretion, or by dramatic eruptive events
in connection with mass outflow) producing observable intensity vari
ations through all wavelengths. Many of the papers in this volume
are concerned with galactic stars showing such behaviour (for ex
ample cataclysmic variables, symbiotic stars, luminous blue variables
(LBVs) and novae). Intrinsic variations of the, magnetic field struc
ture on observable time scales are, for instance, a peculiarity of cool,
solar-like stars and represent a useful tool to study differential rotation
and cycles of activity.
Recent results on the investigation of gas motions in the inner
regions of the Milky Way system, a study of the bipolar galaxy M 82,
observations of variable extragalactic radio sources, the variability of
emission lines in active galactic nuclei (AGN) as well as continuum
variations of quasars and AGN are discussed by several authors.
The yearbook series Reviews in Modern Astronomy of the AG
was established in 1988 in order to bring the scientific events of the
meetings of the society to the attention of the worldwide astronomical
community.
Volume 5 comprises all eighteen invited reviews and highlight
contributions presented at the Bamberg meeting by leading scientists
reporting on recent progress and scientific achievements at their re
search institutes.
Heidelberg, February 1992 G. Klare
Contents
Luminous Blue Variables; Quiescent and Eruptive States
By B. Wolf (With 12 Figures) ..................... 1
On Pulsations of Luminous Stars
By A. Gautschy (With 4 Figures) ................... 16
Cataclysmic Variables - Selected Problems
By G.A. Richter (With 12 Figures) .................. 26
Symbiotic Stars
By R. Luthardt (With 9 Figures) 38
Abundances of Classical Novae
By J. Andrea (With 7 Figures) ..................... 58
Recent Advances in Studies of the Nova Outburst
By S. Starrfield (With 5 Figures) ................... 73
Accretion Disc Phenomena
By J .E. :Pringle " . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
The Variability of Magnetic Stars
By J.D. Landstreet ............................. 105
Observational Aspects of Stellar Seismology
By D. Baade (With 2 Figures) ..................... 125
Testing Stellar Evolution Theory
with Oscillation Frequency Data
By W. Dziembowski (With 6 Figures) 143
Evolution of Stars and Gas in Galactic Nuclei
By R. Spurzem (With 3 Figures) . . . . . . . . . . . . . . . . . . .. 161
Gas Motions in the Inner Galaxy and
the Dynamics of the Galactic Bulge Region
By O.E. Gerhard (With 3 Figures) .................. 174
Stellar X-Ray Variability as Observed with the ROSAT XRT
By J.H.M.M. Schmitt (With 8 Figures) ............... 188
M82 - The Bipolar Galaxy
By P. Notni (With 8 Figures) ...................... 200
Variability and VLBI Observations
of Extragalactic Radio Sources
By A. Quirrenbach (With 12 Figures) 214
Emission Line Variability in AGN's
By W. Kollatschny (With 19 Figures) 229
The Continuum of Quasars and Active Galactic Nuclei,
and Its Time Variability
By M.-H. Ulrich (With 3 Figures) ................... 247
Gravitational Lensing by Large-Scale Structures
By M. Bartelmann (With 10 Figures) . . . . . . . . . . . . . . . .. 259
Index of Contributors. . . . . . . . . . . . . . . . . . . . . . . . . .. 271
Luminous Blue Variables;
Quiescent and Eruptive States
B. Wolf
Landesstemwarte Konigstuhl, W -6900 Heidelberg, FRG
Abstract: The Luminous Blue Variables (LBVs) represent a short-lived (10
000 to 20 000 yrs) episode of the late evolutionary phases of massive stars.
At minimum the LBVs define an inclined instability strip in a range of abso
lute magnitudes and temperatures of - 8 ;: M ~ - 10 and 10 000 ~ T ~ 35 000
K, respectively. The most luminous LBVs are the hottest ones. The spectra
of LBVs in. minimum phase show (dependent on luminosity) characteristics
of Ofpe/WN9 transition type stars and P Cygni-type stars. At outburst
the stars are surrounded by slowly (v ~ 100 to 200 km s-1) expanding,
dense (Ne ~ 1011 cm -1) envelopes, typically exhibiting an equivalent spec
tral type of middle A, but with strong P Cygni-type profiles of HI and of
singly ionized metals. During the last 10 to 15 years a lot has been learnt
about the physical nature of the LBVs and their behaviour during brighten
ing (typically V ,.... 0.5 - 2 mag in times cales of decades) from photometric
j
and high resolution spectroscopic (ground based and IUE) long-term moni
toring programs. Within these programs further instances of LBVs , both in
the Galaxy and in the LMC have been detected. Results of the present un
derstanding of the LBVs at quiescence and eruptive states mainly obtained
from these studies are reviewed.
1. Introduction
The first discovered Luminous Blue Variable was P Cygni. The historical
light curve of P Cygni is shown by Figure 1 (d. Zinner, 1952). It had an
outburst in 1600; a second outburst of shorter duration of about three mag.
was observed more than fifty years later (Fig.l). P Cygni is now a rarther
stable luminous early B supergiant exhibiting only minor photometric vari
ations of 0.1 to 0.2 mag which are not unusual for hot supergiants. Thus, P
Cygni can be classified as a hibernating LBV.
Well investigated active LBVs are S Dor, R 71 and R 127 of the LMC.
Their light variations in the visual range during the last 10 to 25 years are
shown in Figures 2, 3 and 4. LBVs of the LMC have previously often been
designated as S Dor variables.
2
--
n _P_ Cyg
~ /~
~~--------
6-
1600 1700 1800 1900
Fig. 1. Historicallightcurve of P Cyg (cf. Zinner 1952)
Since LBVs are apart from supernovae during outburst the most lu
minous stars in the Universe they have been detected in the Local Group
galaxies M31 and M33 by Hubble and Sandage (1953) and were previously
denoted as Hubble-Sandage variables.
Even beyond the Local Group a number of LBVs were detected in M81
(Sandage, 1984), NGC 2403 (Tammann and Sandage, 1968), and MI0l
(Sandage, 1983).
S Dor
'''l.July 18
v;- .j.
UI ~ o
-020 o
>: o
o
o
&&00 6000 6500 7000 7500 6000
JD244- (days)
Fig.2. Visual different.iallight curve of S Dor of the LMC (observation from Stcrkcn's
LTPV group, St.erken (1983)). The y-magnitude of Cl is 9.19. Arrows indicat.e cpochs
uf CASPEC observations. Note the brightness of 1988 which represents the historical
maximulll of S Dor.
LBVs are as bright as Mv = -9 to -11. They exhibit irregular photometric
variations of 1 to 2.5 mag in timescales of years, decade~z,or longer. These
are the typical amplitudes of normal LBV outbursts. Giant eruptions of
an amplitude of about 6 mag like in the case of TJ Car about 150 years
ago are outstanding and very scarce and are not specifically discussed in
this review (but see e.g. Davidson, 1989, Hillier, 1991). Although the visual
3
brightness during the "normal" outburst increases typically by more than
one magnitude the bolometric luminosity remains essentially unchanged;
LBVs are cooler when brighter (d. e.g. Wolf, 1989). At quiescence LBVs
show (peculiar) early type spectra (OB to early A), during outburst they
exhibit typically middle A to early F spectra. The mass loss rates are high
of the order of 10-5 to 10-4 M0 yr-1. Since the wind velocities are low (100
to 200 km s-1 at maximum and somewhat higher at minimum) LBVs are
characterized by very dense winds.
LBVs are of interest in several respects. Due to their enormous brightness
they can in principle be useful for measuring the cosmological distance scale.
For this a reliable understanding of the main physical properties of LBVs
is an important prerequisite.
LBVs are particularly important in connection with present-day evolu
tionary models of very massive stars. According to a widely accepted sce
nario they represent a late, short-lived phase prior to becoming WR-stars.
The unique wind characteristics mentioned above make LBVs ideal lab
oratories fqr studying slow, dense cool winds around hot stars.
R 71
10 ".>~, .
I ..
.
:.;.:
10. ,
'r\
I
j
. '. ,:1,., .
f; :" ,., ..
. ' .
II
11.1~L"_~~I...J,L...70~~-I-'''-'~~~I9..LI-O ~--U.L.I~'~ -'--I-'''O
Fig.3. Light curve of the LMC-LBV R 71. The observations prior to 1978 were taken
from van Genderen (1977). Observations after 1982 are from the "Long-term photometry
of Variables" (LTPV) group, initiated by Sterken (1983).
A major aim of this review is to discuss the main properties of LBV s
both at quiescence and at outburst. Finally I will discuss new spectrosfopic
observations of S Dor and R 127 of the LMC which may provide new insights
in the outburst behaviour of LBVs. Other more recent reviews with empha
sis on partly different aspects can be found in Lamers (198'1'), Humphreys
(1989), Wolf (1989a), Stahl (1990) and Hillier (1991).