CH Cyg Research Articles

Millimetre observations of the S-type AGB star chi Cygni: Variability of the emission of the inner envelope

New observations are presented of millimetre line emissions of the circumstellar envelope (CSE) of the asymptotic giant branch (AGB) star chi Cygni, using the recently upgraded NOEMA array. chi Cygni is an S-type Mira variable, at the border between oxygen-rich and carbon-rich stars. It has been observed for over 40 years to display features that suggest evidence for the strong role played by pulsation-associated shock waves in the generation of its wind. These new observations provide evidence of a bright H12CN(3-2) line emission confined to the very close neighbourhood of the star; however, this emission appears significantly more extended in 2024 than in 2023. The interpretation of such variability in terms of maser emission has been considered and found to raise significant unanswered questions. Moreover, other unexpected features are observed in the very close neighbourhood of the star, including low Si16O(6-5)/Si17O(6-5), 28SiO(5-4)/29SiO(5-4), and 12CO(2-1)/13CO(2-1) line emission ratios. We discuss several features, which possibly confirm the important role played by shocks: a measurement of the SiO(5-4)/SiO(6-5)emission ratio; the observation of a recent mass ejection, particularly enhanced in the north-western red-shifted octant, which has left a depression in its wake; patterns of enhanced CO(2-1) line emission, which suggest an interpretation in terms of episodic outflows, on a time scale of a few decades, enhanced over solid angles associated with the surface of convective cells. Unravelling the mechanisms underlying such newly observed features is very challenging. Thus, a confirmation of the reported observations with improved sensitivity and angular resolution would be highly welcome. The observation of SiO maser emission in the (nu =1,$J$=6-5) transition is reported for the first time.

On the impact of «red leak» of filters on brightness estimates of late stars using the example of observations of rapid variability of symbiotic stars

The results of modeling the dependence of the «red leak» of photometric filters on various factors (color index 𝑉 −𝑅, luminosity class, interstellar reddening, air mass and PWV) during observations of stars are presented. The error arising from not taking into account the «red leak» in the case of filters used on the 0.6-m telescope of the CMO SAI can amount to 0.6 𝑚 − 0.8 𝑚 for late stars. Algorithms for reducing observational data are presented for filters 𝑈 and 𝐵. The results of observations of the rapid variability of two symbiotic stars CH Cyg and SU Lyn with cold components of very late spectral types are presented. For CH Cyg, rapid variability was detected on both observation dates. Taking into account the «red leak» effect, the amplitude in the 𝐵 band was 0.10𝑚 on November 6, 2019 and 0.19𝑚 on December 15, 2022, with a characteristic variability time of about 20 minutes. For SU Lyn, no rapid brightness variability was detected in the 𝐵 band on February 2, 2023 (with an accuracy of 0.003𝑚)

THE Hα AND Hβ LINES IN THE SPECTRUM OF CH CYG IN 2016

We present the results of spectral observations of the symbiotic star CH Cyg, carried out at the Cassegrain focus of the 2-m telescope of the ShAO named after N. Tusi, by using the Shamakhy Fiber Echelle Spectrograph (ShAFES). The spectra of star CH Cyg were obtained with the spectral resolution of R = 28000, between June and November 2016. We also present the results of the comparative analysis of the brightness curve of the star with the main parameters of Hα and Hβ emission lines. During the observation period, both lines exhibited changing profiles characterized by complex structures, predominantly with double peaks. The ratio of the intensities of the blue and red components was typically IB/IR < 1 for the Hα line and IB/IR ≥ 1 for the Hβ line. A quasi-period of 241 days was found in the variation of the equivalent widths of Hα and Hβ emission lines as well as in the intensities of their components.

Evolution in pulsating variable stars: Long term, and inter-cycle

Abstract We used machine learning techniques to predict period changes in variable stars using noisy and sparse time series data, while inferring underlying physics and generalizing predictions about cycle-to-cycle variations. Our focus was on Mira variables, a well-known class of pulsating stars. Pre-processing data from Mira, R Andromedae, U Orionis, and Chi Cygni, obtained from the American Association of Variable Star Observers (AAVSO), we predicted luminosity magnitude uncertainty and classified pulsation states. Employing various classification and regression algorithms, along with feature engineering, we aimed to generalize predictions. We created a generalized dataset with collective averaged data points, limiting our analysis to a common time duration. Linear regression models yielded no successful predictions, but Decision Tree and KNN regressors accurately predicted luminosity magnitude errors, indicative of variation over time. Feature engineering successfully aided regression and classification of pulsating star states. After hyper parameter tuning using Bayesian Neural networks, we achieved a classification accuracy of 0.8 and 0.94 for the KNN classifier, respectively, in classifying pulsation states of Mira variables. The regression model achieved an R2 score of 0.98. Our work provides a foundation for developing tools to analyze various pulsating star variables, including Cepheids, RR Lyrae, and Delta Scuti variables, as well as other astrophysical data. These techniques demonstrate impressive performance with time series datasets.

Peering into the central region of a nano-quasar: XMM–Newton and Chandra views of the CH Cyg symbiotic system

ABSTRACT We present the analysis of archival XMM–Newton and Chandra observations of CH Cyg, one of the most studied symbiotic stars (SySts). The combination of the high-resolution XMM–Newton RGS and Chandra HETG X-ray spectra allowed us to obtain reliable estimates of the chemical abundances and to corroborate the presence of multitemperature X-ray-emitting gas. Spectral fitting of the medium-resolution XMM–Newton MOS (MOS1+MOS2) spectrum required the use of an additional component not seen in previous studies in order to fit the 2.0–4.0 keV energy range. Detailed spectral modelling of the XMM–Newton MOS data suggests the presence of a reflection component, very similar to that found in active galactic nuclei. The reflection component is very likely produced by an ionized disc (the accretion disc around the white dwarf) and naturally explains the presence of the fluorescent Fe emission line at 6.4 keV while also contributing to the soft and medium energy ranges. The variability of the global X-ray properties of CH Cyg are discussed as well as the variation of the three Fe lines around the 6–7 keV energy range. We conclude that reflection components are needed to model the hard X-ray emission and may be present in most β/δ-type SySt.

Spectrophotometric Monitoring of the Activity of the Symbiotic Star CH Cyg from 2008 to 2018

Spectrophotometric Monitoring of the Activity of the Symbiotic Star CH Cyg from 2008 to 2018

Detection of Periodicity in SiO Maser Intensity and Velocity Shift of the Symbiotic Star CH Cyg

Abstract We performed simultaneous monitoring observations of H2O 616–523 and SiO v = 1, 2, 0, SiO v = 1, 1, maser lines toward the symbiotic star CH Cyg from 2012 December to 2019 November using the Korean very long baseline interferometry Network single-dish telescope. SiO v = 1, 2, and v = 1, maser lines were detected for the first time. A periodicity of about 2.5 yr in the SiO maser intensity was found, showing its association with the pulsation of the M giant in CH Cyg. Only the SiO v = 2, maser, without detections of the v = 1, maser, was detected in 6 epochs out of 15 v = 2, J = 1–0 maser detected epochs. In addition, the SiO v = 1, maser intensity was always stronger than that of the v = 1, maser except for one epoch, providing a critical constraint on the different masing conditions by different transitions, according to changes of the CH Cyg symbiotic system. We also found the gradual redshifted peak velocities of the SiO masers had about a 2.5 yr periodicity corresponding to the period of peak intensities (redshifted up to about with respect to the stellar velocity of the M giant). Two possible explanations can be suggested for the 2.5 yr period of the SiO maser intensity and velocity behaviors, although their actual causes remain uncertain.

High-velocity equatorial mass ejections and some other spectroscopic phenomena of the symbiotic star CH Cygni in an active stage

Context. CH Cyg is one of the most studied symbiotic stars. Its properties, however, are still not well known. Two main periods, about 15 years and 750 days, are known in the photometric and spectroscopic variations, and two models are proposed for these origins. One is a binary system with an orbital period of 15 years consisting of a hot component and pulsating red giant with a 750-day period. The other is a triple system consisting of an inner symbiotic binary with an orbital period of about 750 days and third component with an orbital period of 15 years. Several active stages have been observed since the 1970s during which the object brightened up by ΔU = 3−5 mag and prominent emission lines appeared. Large mass outflows were observed at some active stages. Aims. The spectral variation of CH Cyg has been monitored at Asiago Observatories to understand the problems mentioned above. We have analysed spectra obtained in the time period from 1995 to 2004 which covers an active stage during the years 1998−2000. Methods. High- and low-resolution optical spectra obtained at the Asiago Observatories are used. Results. Narrow absorption lines of Fe I, Cr I, Ti I, and so on appeared in 1998 at an early phase of the active stage. These lines are clearly distinguished from those of the M-type giant and are typically found on the spectrum of early A-type dwarfs. They were redshifted by about 30 km s−1 with respect to the absorption lines of the M-type giant. Assuming that their radial velocities represent the orbital motion of the hot component, its semi-amplitude is estimated to be 37.0 ± 0.5 km s−1. The masses of the hot component and the M-type giant are estimated to be 0.32 ± 0.02 M⊙ and 4.6 ± 0.2 M⊙, respectively, where a circular orbit with a period of 756 days is adopted. If the inner binary system has an elliptical orbit, e = 0.33, and a period of 750.1 days, the masses of the two components are 0.21 ± 0.01 M⊙ and 2.2 ± 0.1 M⊙, respectively. Our results lend support to the triple system model, because if the period of the symbiotic binary were 15 years, the mass of the hot component would be expected to exceed the Chandrasekhar limit. Highly blueshifted absorption components of H I and He I lines appeared at a later phase of the active stage. Mass ejections with velocities on the order of 1000 km s−1 seem to have occurred along the orbital plane from December 1998 to March 1999. The highest outflow velocity, − 2383 km s−1, was observed on 1999 February 26. Narrow absorption components of Na I D1, D2, and Fe II lines redshifted by 10−15 km s−1 coexisted with the highly blueshifted broad absorption components of H I and He I lines. This phenomenon might have been related to an inner disc inflow expected in wind-compressed discs. In contrast to the bipolar mass outflows at the past active stages, high-velocity equatorial mass ejections likely occurred at the active stage during the years 1998−2000. There should have been an eclipse of the hot component by the M-type giant in the inner binary system in the time period of December 1998 to January 1999. A clear light curve of the eclipse, however, was not detected. Possibly, the luminosity of the hot component was due mainly to free-free emission from the ejected circumstellar matter which was likely more extended than the M-type giant. On the other hand, another eclipse by the third component with the period of 15 years began at the end of May 1999 during which the hot component as well as the emitting regions of Hβ and Fe II lines were well eclipsed. The obscuring matter around the third component should have been much more extended than the M-type giant, and it was likely semi-transparent, because the spectrum of the M-type giant was well seen during the eclipse. The third component appears to be similar to the invisible secondary component in the long-period eclipsing binary ϵ Aur.

Broad absorption line symbiotic stars: highly ionized species in the fast outflow from MWC 560

In symbiotic binaries, jets and disk winds may be integral to the physics of accretion onto white dwarfs from cool giants. The persistent outflow from symbiotic star MWC 560 (=V694 Mon) is known to manifest as broad absorption lines (BALs), most prominently at the Balmer transitions. We report the detection of high-ionization BALs from C IV, Si IV, N V, and He II in International Ultraviolet Explorer spectra obtained on 1990 April 29-30, when an optical outburst temporarily erased the obscuring 'iron curtain' of absorption troughs from Fe II and similar ions. The C IV and Si IV BALs reached maximum radial velocities at least 1000 km/s higher than contemporaneous Mg II and He II BALs; the same behaviors occur in the winds of quasars and cataclysmic variables. An iron curtain lifts to unveil high-ionization BALs during the P Cygni phase observed in some novae, suggesting by analogy a temporary switch in MWC 560 from persistent outflow to discrete mass ejection. At least three more symbiotic stars exhibit broad absorption with blue edges faster than 1500 km/s; high-ionization BALs have been reported in AS 304 (=V4018 Sgr), while transient Balmer BALs have been reported in Z And and CH Cyg. These BAL-producing fast outflows can have wider opening angles than has been previously supposed. BAL symbiotics are short-timescale laboratories for their giga-scale analogs, broad absorption line quasars (BALQSOs), which display a similarly wide range of ionization states in their winds.

Active Stage of the Object CH Cyg B in 2014-2015

The results of spectral and photometric observations of the emission object CH Cyg are presented. The active stage of CH Cyg during 2014-2015 showed up as an increase in the brightnesses to B~7m.8 and V~6m.8, intensification of the continuum, and increased fluxes in emission lines. A rapid, within days, variability in the emission line profiles was observed. The velocities of individual absorption components were as high as ~3000 km/s.

Flickering of accreting white dwarfs: the remarkable amplitude–flux relation and disc viscosity

Abstract We analyse optical photometric data of short term variability (flickering) of accreting white dwarfs in cataclysmic variables (KR Aur, MV Lyr, V794 Aql, TT Ari, V425 Cas), recurrent novae (RS Oph and T CrB) and jet-ejecting symbiotic stars (CH Cyg and MWC 560). We find that the amplitude–flux relationship is visible over four orders of magnitude, in the range of fluxes from 1029 to 1033 erg s−1 Å−1, as a ‘statistically perfect correlation with correlation coefficient 0.96 and p-value ∼10−28. In the above range, the amplitude of variability for any of our 9 objects is proportional to the flux level with (almost) one and the same factor of proportionality for all nine accreting white dwarfs with ΔF = 0.36(±0.05)Fav, σrms = 0.086(±0.011)Fav, and σrms/ΔF = 0.24 ± 0.02. Overall, our results indicate that the viscosity in the accretion discs is practically the same for all nine objects in our sample, in the mass accretion rate range 2 × 10−11 − 2 × 10−7 M⊙ yr−1.

Rapid Photometric Variability Of The Symbiotic System CH Cyg During 2008–15

Our photometric monitoring of CH Cyg revealed a gradual increase of its brightness in U from 2010. More recent observations, made during 2014–2015 suggest that CH Cyg entered a new active phase. Monitoring for rapid variability demonstrated unambiguous connection between appearance of pronounced rapid variations and the level of the overall brightness in U , i.e. the activity of CH Cyg.

Fe Kα line in hard X-ray emitting symbiotic stars

The 6.4 keV iron emission line is typically created by irradiation of the neutral (or low ionized) iron by a hard X-ray source. Whereas the 6.7 and 7.0 keV emission lines are mainly produced by photoionization and collisional excitation in hot plasma, the 6.4 keV fluorescence line is typically a signature of either reflection from an accretion disc or absorption. We have surveyed the emission using a collection of Suzaku observations of hard X-ray emitting symbiotic stars (hSSs) to better understand the geometry of these systems. We find that they do not seem to have a single geometry, and that while absorption-induced fluorescence leads to some emission in three of the hSSs in our study, CH Cyg, T CrB and RT Cru, there are strong hints that significant 6.4 keV emission arises in the accretion disc irradiated by the hard X-rays from the boundary layer between the accretion disc and hot white dwarf in one of our sources, SS73 17. The 6.7 and 7.0 keV lines, however, are largely produced by collisional excitation in the vicinity of the compact white dwarf.

Photometric Activity of the Symbiotic Star CH Cyg During 2008–2011

Abstract We present results of UBVRC observations of the symbiotic star CH Cyg before and after optical and X-ray bursts in 2009 at various timescales. Rapid variability was found only during a strong burst (U ~ 9.5 mag) in 2009 October, while during the low state, prior to (U ~ 11 mag) and after (U ~ 9.4 mag) the burst rapid variability was not detected.

Recent photometry of symbiotic stars

AbstractWe present new multicolour (UBVRCIC) photometric observations of classical symbiotic stars, EG And, Z And, BF Cyg, CH Cyg, CI Cyg, V1329 Cyg, TX CVn, AG Dra, Draco C1, AG Peg, and AX Per, carried out between 2007.1 and 2011.9. The aim of this paper is to present new data of our monitoring programme, to describe the main features of their light curves (LC) and to point problems for their future investigation. The data were obtained by the method of the classical photoelectric and CCD photometry (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Search for and study of hot circumstellar dust envelopes

Search for and study of hot circumstellar dust envelopes

EVIDENCE FOR THE WHITE DWARF NATURE OF MIRA B

The nature of the accreting companion to Mira --- the prototypical pulsating asymptotic giant branch star --- has been a matter of debate for more than 25 years. Here we use a quantitative analysis of the rapid optical brightness variations from this companion, Mira B, which we observed with the Nickel telescope at Lick Observatory, to show that it is a white dwarf (WD). The amplitude of aperiodic optical variations on time scales of minutes to tens of minutes (approximately 0.2 mag) is consistent with that of accreting WDs in cataclysmic variables on these same time scales. It is significantly greater than that expected from an accreting main-sequence star. With Mira B identified as a WD, its ultraviolet (UV) and optical luminosities, along with constraints on the WD effective temperature from the UV, indicate that it accretes at ~1e-10 solar masses per year. We do not find any evidence that the accretion rate is higher than predicted by Bondi-Hoyle theory. The accretion rate is high enough, however, to explain the weak X-ray emission, since the accretion-disk boundary layer around a low-mass WD accreting at this rate is likely to be optically thick and therefore to emit primarily in the far or extreme UV. Furthermore, the finding that Mira B is a WD means that it has experienced, and will continue to experience nova explosions, roughly every million years. It also highlights the similarity between Mira AB and other jet-producing symbiotic binaries such as R Aquarii, CH Cygni, and MWC 560, and therefore raises the possibility that Mira B launched the recently discovered bipolar streams from this system.

CO observations of symbiotic stellar systems

We have performed mm-wave observations with the IRAM 30m telescope of the 12CO J=2-1 and J=1-0, 13CO J=2-1 and J=1-0, and SiO J=5-4 transitions in the symbiotic stars R Aqr, CH Cyg, and HM Sge. The data were analyzed by means of a simple analytical description of the general properties of molecular emission from the inner shells around the cool star. Numerical calculations of the expected line profiles, taking into account the level population and radiative transfer under such conditions, were also performed. Weak emission of 12CO J=2-1 and J=1-0 was detected in R Aqr and CH Cyg; a good line profile of 12CO J=2-1 in R Aqr was obtained. The intensities and profile shapes of the detected lines are compatible with emission coming from a very small shell around the Mira-type star, with a radius comparable to or slightly smaller than the distance to the hot dwarf companion, 10$^{14}$ - 2 10$^{14}$ cm. We argue that other possible explanations are improbable. This region probably shows properties similar to those characteristic of the inner shells around standard AGB stars: outwards expansion at about 5 - 25 km/s, with a significant acceleration of the gas, temperatures decreasing with radius between about 1000 and 500 K, and densities ~ 10$^9$ - 3 10$^8$ cm$^{-3}$. Our model calculations are able to explain the asymmetric line shape observed in 12CO J=2-1 from R Aqr, in which the relatively weaker red part of the profile would result from selfabsorption by the outer layers (in the presence of a velocity increase and a temperature decrease with radius). The mass-loss rates are somewhat larger than in standard AGB stars, as often happens for symbiotic systems. In R Aqr, we find that the total mass of the CO emitting region is ~ 2 - 3 10$^{-5}$ Mo, corresponding to M' ~ 5 10$^{-6}$ - 10$^{-5}$ Mo/yr, and compatible with results obtained from dust emission. Taking into account other existing data on molecular emission, we suggest that the small extent of the molecule-rich gas in symbiotic systems is mainly due to molecule photodissociation by the radiation of the hot dwarf star.

First spatial resolution of the stellar components of the interacting binary CH Cygni

Abstract We report the first resolved bispectrum speckle interferometry of the symbiotic binary CH Cyg. The measured component separation, ρ= 42 ± 2mas, is consistent with the one derived from the known spectroscopic orbit and distance. In particular, our result implies a total mass of the binary of Mt=Mg+Mwd= 3.7+3.5−1.7M⊙, which is in good agreement with the value Mt= 2.7+1.2−0.6M⊙ derived from the spectroscopic orbit solution for the red giant and evolutionary constraints. We also show that the radio jets and the bipolar outflow are not orthogonal to the orbital plane of the binary system.

INFRARED SPECTROSCOPY OF SYMBIOTIC STARS. VIII. ORBITS FOR THREE S-TYPE SYSTEMS: AE ARAE, Y CORONAE AUSTRALIS, AND SS 73-147

With new infrared radial velocities we have computed orbits of the M giants in three southern S-type symbiotic systems. AE Ara and SS 73-147 have circular orbits with periods of 803 and 820 days, respectively. The eccentric orbit of Y CrA has a period that is about twice as long, 1619 days. Except for CH Cyg it is currently the S-type symbiotic system with the longest period for which a spectroscopic orbit has been determined. The Paschen δ emission line velocities of AE Ara are nearly in antiphase with the M giant absorption feature velocities and result in a mass ratio of 2.7. Emission lines in the 1.005 μm region for the other two symbiotic systems are not good proxies for the hot components in those systems. There is no evidence that these three symbiotics are eclipsing. With spectral classes of M5.5 or M6, the three giants presumably also have velocity variations that result from pulsations, but we have been unable to identify specific pulsation periods in the absorption line velocity residuals.