Radio Astronomy Department
Sternberg Astronomical Institute, Moscow University

Multiwavelength Study of Mira Ceti-Type Variable Stars

The Goals of the Project

At present of great importance in astrophysics is the study of late evolutionary stages of intermediate-mass stars (of the order of 1-3 solar masses), in particular, the stars on the asymptotic giant branch (AGB). At this stage, stars intensely lose mass and, in about a hundred thousand years, pass from the red-giant stage to the stage 'white dwarf + planetary nebula'. This transition results in radical changes in star's structure. The matter, lost by the giant stars, forms thick gas-dust envelopes around the stars. Later, this matter goes to the interstellar medium, exerting a strong influence on the composition of the latter, especially on the interstellar dust. The AGB red giants are subject to pulsational instability; most of them are variable stars, belonging to the Mira Ceti type or to the semiregulars, with timescales of brightness variations of a few hundreds of days.

We are interested with the effects, connected with AGB stars' variability; in particular, with the phenomena due to passage of shock waves in the atmospheres and envelopes of the stars (shock waves are formed during each cycles of stars' light variations). The appearance of shock waves is signaled by presence of emission lines of hydrogen (Balmer series) and metals in stars' spectra. According to theoretical concepts, passage of shock waves should also cause radio continuum emission, both thermal and nonthermal (connected with acceleration of electrons on the shock front). Shock-induced nonthermal processes may lead to rapid (at timescales of a few hours) light variations within a few tenths of a stellar magnitude.

According to their chemical composition, red giant variables form two big groups: oxygen-rich and (less numerous) carbon-rich, of equal interest for us; oxygen-rich stars are also distingusihed with circumstellar maser emission of the OH, H₂O, and SiO molecules.

Shock waves may influence the intensity of maser emission of the OH, H₂O, and SiO in circumstellar envelopes - both directly (by impact of the shock on the maser generation region) and indirectly (through the radio continuum emission of the shock, amplified by masering molecules). There are reasons to suspect such a correlation.

Background and Significance

There have been a number of studies on late-type variables in the UBV bands, infrared, and in molecular lines. However, we plan here a long-term, multiwavelength study of a sample of the stars, covering a few cycles of their variability. This will enable us to obtain complete time series of observational data in several spectral ranges. A comparison of these data would yield a new insight to the problem of late-type stars' variability, mass loss, maser emission, and circumstellar envelopes.

Preliminary Studies

The team possesses equipment necessary for the project: UBV and IR photometers, 100-cm and 60-cm telescopes of the Sternberg Institute Observatory in the Crimea, multichannel radio spectrum analyzer on the 22-meter radio telescope in Pushchino. There are preliminary series of photometric and radio spectral observations of the above-mentioned stars, archival data on the light curves of the stars, results of the search for OH and H₂O maser emission in IRAS point sources that possess the IR parameters similar to Miras. We studied theoretically the connection between stars' visual and IR brightness with the intensity of circumstellar radio emission. We suggested observational tests for the models developed.

Research Design and Methods

We carry out the following observational programs within the framework of the project.

(1) Regular optical (UBV) monitoring of several Mira-type and semiregular variables (o Cet, chi Cyg, R Aql, R Leo, U Ori, U Her, RT Vir, R Cas, IK Tau, R Crt, RX Boo, W Hya; the list will be extended). The aim is to construct detailed light curves of the stars and to search for rapid light variations at some time intervals. The pre-maximum phases of the light curves deserve special attention, because at this time a developed shock is present in the stellar envelope, and strong nonthermal bursts are possible.

(2) Optical spectroscopy of the same stars at the phases, when emission lines of hydrogen and metals are present. Of special interest would be detection of rapid emission-line variability in Miras, as well as observations of emission lines in semiregular variables (for which there are only fragmentary data on emission-line observations).

(3) Radioastronomical spectral observations of maser emission of the same stars in 1.35-cm line of the H₂O molecule. The aim is a detailed study of the maser emission variability. A search for new masers toward IR stars from the IRAS Point Source Catalog is carried out.

(4) IR observations of the stars, aiming to study the amplitude and character of the bolometric luminosity variations of the Miras; evaluation of the dust envelope parameters; tracing the correlation between IR and maser emission.

(5) Observations of the stars at centimeter wavelengths for a systematic search of the radio continuum emission, connected with the shock waves, and of the influence of the shocks and of the continuum on the molecular maser emission.

(6) Compilation and analysis of data on the light curves, optical spectroscopy, continuum and molecular maser emission for a broader sample of Miras and semiregular variables.

We carry out the spectral observations (2) by means of the grating spectrograph with CCD cameras ST-6, ST-6V, ST-8, and ST-402 manufactured by the Santa Barbara Instruments Group. This instrumentation has already been used in observations on the 70-cm telescope in Moscow and on 100- and 60-cm telescopes in the Crimea. In August 1997, test observations with an echelle spectrograph (resolving power 10,000) were performed in the Crimea. Below, a raw echelle spectrum of o Cet is shown.

Radio observations in the H₂O line (3) are performed since 1980 on the 22-meter radio telescope of the Pushchino Radio Astronomy Station, Astrospace Center, Lebedev Physical Institute, Moscow Region, by means of the 1.35-cm wavelength receiver with a cooled FET preamplifier and a 128-channel filter-bank spectrometer, designed and constructed in the Sternberg Institute.

Radio continuum observations of miras (5) were carried out jointly with Dr. J.M.Chapman ( Australia Telescope National Facility) in November 1995 on the Australia Telescope Compact Array at 4.8 and 8.64 GHz. Out of 34 miras, only R Aqr was detected, the rest have upper limits between 0.16 and 0.66 mJy at both frequencies.

IR observations (4) are carried out on the Crimean telescopes, equipped with the IR photometer of the Sternberg Astronomical Institute.

Team Members

Dr. ESIPOV, Valentin F., Project Manager (optical spectroscopy)
Dr. NADJIP, Aurengzeb E. (infrared observations)
Dr. LEKHT, Evgueny E. (H₂O observations, radio data reduction)
Dr. PASHCHENKO, Mikhail I. (H₂O observations)
ROTTAR, Svetlana V. (optical spectroscopy, spectral data reduction)
Dr. RUDNITSKIJ, Georgij M. (H₂O observations, radio data reduction, theory)

Recent Results

In 1996-1997, astronomical observations of Mira Ceti-type stars (miras) were carried out in three spectral ranges: optical, infrared, and radio. The main sample consisted of 12 stars; we also obtained data in various spectral ranges for another 20 stars. We constructed time dependences of visible and near-infrared brightness, measured spectral profiles of circumstellar maser radio emission in the H₂O molecule line at 1.35 cm, obtained series of spectra at visible wavelengths. We reduced the observations and compared the data obtained in different spectral ranges. See sample curves for R Leo below. In summer 1996, the H_alpha emission was quite strong; though, the H₂O emission through 1994-1997 was below our detection limits of about 10 Janskys.

The correlation of the intensity variations of the H_alpha hydrogen emission line and H₂O maser with optical variability in maser stars U Her, R Aql, RR Aql, W Hya, U Ori, R Cas and others was confirmed. There is a phase delay of the maser variability curve with respect to the optical and infrared curves by about 0.1-0.3 of optical period P. For the star RR Aql, the delay is as long as 0.5P. For the star U Ori, we confirmed the superperiod of the maser variability. We estimated parameters of gas in the maser layer of the circumstellar shell and velocities of shock waves (for different stars, between 6 and 20 km/s). We considered manifestations of asymmetry of circumstellar shells in profiles of the H₂O maser emission. We developed theoretical models of circumstellar radio emission in the continuum and H₂O maser line, taking into account the effects of shock waves. Novelty of the work performed is in that there were obtained homogeneous series of multiwavelength observations; this enabled us to connect, in a unified preliminary model, properties of the circumstellar masers with parameters of stars' visual-light variability and to distinguish the common cause of the variation - shock waves originating from stellar pulsations. The results obtained give us an opportunity to further study of stellar pulsations at late evolutionary stages by both optical and radio techniques, thus providing a new approach to the fundamental problem of pulsational instability of the red giant stars.

Project Manager
V.F.Esipov