XX Oph Research Articles

For optical flickering in symbiotic star MWC 560

Abstract This study is based on observations of MWC560 during the last two observational seasons (2020/2021 and 2021/2022). Other than looking for flickering we were interested in following the variability of brightness in the same period. Looking for similarities in the spectra with other types of stars is also of great interest to us because it could help clarify the stellar configuration of such objects. Our observations during the last two observational seasons of MWC560 confirm the absence of flickering. From the similarities of the gathered spectra of XX Oph and MWC560 we assume that the components in XX Oph are a red giant and a white dwarf, which are also surrounded by a common shell.

Solid-phase C60 in the peculiar binary XX Oph?

ABSTRACT We present infrared spectra of the binary XX Oph obtained with the Infrared Spectrograph on the Spitzer Space Telescope. The data show some evidence for the presence of solid C60– the first detection of C60 in the solid phase – together with the well-known ‘unidentified infrared’ emission features. We suggest that, in the case of XX Oph, the C60 is located close to the hot component, and that in general it is preferentially excited by stars having effective temperatures in the range 15 000–30 000 K. C60 may be common in circumstellar environments, but unnoticed in the absence of a suitable exciting source.

A HIGH-RESOLUTION, MULTI-EPOCH SPECTRAL ATLAS OF PECULIAR STARS INCLUDING RAVE,GAIA, AND HERMES WAVELENGTH RANGES

We present an Echelle+CCD, high S/N, high resolution (R = 20\,000) spectroscopic atlas of 108 well-known objects representative of the most common types of peculiar and variable stars. The wavelength interval extends from 4600 to 9400 Ang, and includes the RAVE, Gaia and HERMES wavelength ranges. Multi-epoch spectra are provided for the majority of observed stars. A total of 425 spectra of peculiar stars are presented, which have been collected during 56 observing nights between November 1998 and August 2002. The spectra are given in FITS format and heliocentric wavelengths, with accurate subtraction of both the sky background and the scattered light. Auxiliary material useful for custom applications (telluric dividers, spectro-photometric stars, flat-field tracings) is also provided. The atlas aims to provide a homogeneous database of the spectral appearance of stellar peculiarities, a tool useful both for classification purposes and inter-comparison studies. It could also serve the planning for and training of automated classification algorithms designed for RAVE, Gaia, HERMES and other large scale spectral surveys. The spectrum of XX Oph is discussed in some detail as an example of the content of the present atlas.

Behind the Iron Curtain: The Environments of the Iron Stars AS 325 and XX Oph

ABSTRACTWe present new photometric and spectroscopic observations of the Iron stars AS 325 and XX Oph. Both stars are binaries composed of a Be star primary plus a luminous red star. Long-term optical photometry of AS 325 reveals it to be a 513-day period eclipsing binary with highly variable eclipses. The primary eclipse is that of the Be star with no secondary eclipse observed. This is not surprising given the small comparative size of the more massive companion. The red star in AS 325 is observed to be a variable star ranging from K2.5 to a M5 in spectral type and shown to be a supergiant of luminosity class II. The red secondary star is the major cause of the variable eclipses. A simple model is advanced to explain the complex behavior observed for Balmer hydrogen emission and Ca II H and K absorption lines as a function of eclipse phase. Spatially resolved near-IR spectroscopy of XX Oph shows the binary to be sitting in a large cavity-like region of space surrounded by reflective dust. AS 325 and XX Oph are nearly identical binary systems.

Lifting the Iron Curtain: Toward an Understanding of the Iron Stars XX Oph and AS 325

ABSTRACTWe present new optical, near‐infrared, and archival ultraviolet observations of XX Ophiuchi and AS 325, two proposed “iron” stars. These unusual stars have optical spectra dominated by emission lines arising from hydrogen, as well as ionized metals such as iron, chromium, and titanium. Both stars have been classified as “iron” stars, and a number of exotic models have been presented for their origin. Using 2 years of moderately high resolution optical spectroscopy, the first high signal‐to‐noise ratio K‐band spectroscopy of these sources (which reveals stellar photospheric absorption lines), and new near‐infrared interferometric observations, we confirm that both systems are composed of two stars, likely binaries, containing a hot Be star with an evolved late‐type secondary. The hydrogen emission features arise in the hot wind from the Be star, while the corresponding P‐Cygni absorption lines are produced from dense material in the expanding, radiation‐driven wind around each system. The optical Fe ii emission lines are pumped by ultraviolet Fe ii absorption lines through fluorescence. Contrary to some claims in the literature, the spectral features of XX Oph and AS 325 are quite similar, evidence that they are comparable systems. We examine the variability of the spectral morphology and radial velocity motions of both sources. We also study the variability of XX Oph during a major photometric event and find that the spectral nature of the system varies during the event. A comparison of the velocity of the absorption‐line components in our new spectra with those in the literature show that the structure of the stellar wind from XX Oph has changed since the system was observed in 1951.

Observations of the peculiar object MWC 560 in outburst

The results of ultraviolet spectroscopy, photoelectric photometry, and supplemental high-resolution H(alpha) spectroscopy of a photometric outburst of MWC 560 are discussed. Ultraviolet spectra are shown to be consistent with the ejection of an optically thick shell that produced strong absorption blends of Fe II and Cr II. The velocities reported exceed by far those previously found in symbiotic stars or recurrent novas. In addition to the variable high-velocity system of broad absorption features, a relatively stable system of Mg II, Mg I, Fe II, Cr II, and other ionic absorptions is observed. It is pointed out that the spectroscopic phenomena in MWC 560 resemble those found in XX Ophiuchi, but the velocities in the MWC 560 are an order of magnitude higher than those found in XX Oph.

The physical properties of the Hα-emission line stars: CD-42°11721, KK Oph and XX Oph

By use of the data found in the literature, observed at the ESO and taken from the IRAS point source catalogue, the physical properties of the Hα-emission stars: CD-42°11721, KK Oph and XX Oph were studied photometrically and spectroscopically. The results can be summarized as follows: (a) Feii and [Feii] emission lines show that these stars have a gaseous envelope of which part is of low density. (b) P Cygni profiles in the spectral lines of XX Oph indicate the existence of rapid gaseous motions in its atmosphere. The presence of P Cygni profiles in the spectra of the other two stars is not certain. (c) In the outer atmosphere of CD-42°11721 and KK Oph at least two circumstellar concentrations of dust grains seems to be present; the temperatures of these dust shells are, for both stars, about 1200 and 250 K. (d) XX Oph has an M6III companion causing the infrared excess. Furthermore, we have studied the physical properties of the dust shells. The distances to the central star, the chemical composition and the masses of each shell were estimated. It is recommended to study above stars further with high dispersion spectroscopy and simultaneous long-term photometry.

AS 325: ANOTHER "IRON STAR"?

ABSTRACT The tenth-magnitude star AS 325, classified as spectral type Fe, is shown to be a remarkable early type emission object. He-alpha is an extremely strong emission line with variable profile. The Na I D lines are in emission, with multiple narrow absorption components. The Ca II IR triplet is in emission. The lines are narrow and have blueshifted absorption components. In the blue the Balmer lines are strong, narrow emission features, and numerous emisison features of Fe II are present. Though there are some differences, the spectrum of AS 325 most closely resembles that of 'Merrill's Iron Star', XX Oph.

Spectrophotometric observations of symbiotic stars and related objects

Calibrated optical spectrophotometric observations of 16 symbiotic and symbiotic-like objects are presented. The objects observed include Z And, T CrB, CH Cyg, CI Cyg, V1016 Cyg, V1329 Cyg, AG Dra, YY Her, RS Oph, XX Oph, AG Peg, AX Per, CL Sco, HM Sge, AS 289, and M1-2. Integrated emission-line intensities are tabulated for comparison with ultraviolet and infrared data, as well as with previous optical studies. The reddening to each of the objects is derived by assuming that Balmer lines are emitted in their case B recombination ratios. However, the values so derived are often systematically higher than reddening estimates from the ultraviolet 2200 A feature. Comparisons with the available data from other wavelength ranges are noted.

THE NEAR-INFRARED SPECTRUM OF XX OPHIUCHI.

ABSTRACT The spectrum of the peculiar emission-line star XX Oph in the spectral interval 6200 A to 9000 A is described. Strengths of two TiO bandheads at 8859 A and 8937 A indicate the spectral type of the cool companion is MS or M6. A list of all the emission lines and absorption lines in the spectrum with probable identifications is given. Key words: spectral type - composite spectrum - binary star

Infrared Lines in Peculiar Emission-Line Stars

An image-tube Cassegrain spectrograph, equipped with an S-1 photocathode enabled us to obtain infrared spectra of peculiar emission-line objects, as V 1016 Cyg, HBV 475, HD 51585, HD 45677, XX Oph and CI Cyg. Many of these objects display emission lines of H I, He I, He II, O I, N I, Ca II [S II] [S III], [Fe II]. The line intensities can be used together with data from other parts of the spectrum to determine relative abundances of these ions in the shells of these objects, which may be at a critical phase of their evolution.

The composite spectrum and energy distribution of XX Oph.

The composite spectrum and energy distribution of XX Oph.