AG Pegasi Research Articles

Spectroscopy of the 2015 Outburst of AG Pegasi

Abstract Between 2015 September and 2016 January, we obtained 18 spectra of the 2015 classical outburst of AG Pegasi—a symbiotic star consisting of a white dwarf (WD), red giant, and surrounding nebula. Modelling the flux contributions of these components reveals that nebular emission, from the reprocessing of high energy WD photons, dominates the 3200–6300 Å range. Nebular emission rises and falls in line with changes seen in the WD, whose properties have been derived using Hβ and He ii (4686 A) line flux, and emission measure calculations. WD parameters follow changes seen in visual band light curves. During the second peak of the outburst, WD temperatures reach 166,000 ± 6000 K, with a luminosity and radius of 14,000 ± 2000 L ⊙ and 0.149 ± 0.014 R ⊙ respectively. These features are consistent with an expansion of the WD pseudo-photosphere due to an accretion rate exceeding that required for stable hydrogen burning.

Photometry of the 2015 Outburst of AG Pegasi

Abstract In 2015 June, the symbiotic binary star AG Pegasi underwent a period of outburst. SkyCam T, mounted onboard the 2 m Liverpool Telescope, collected over 650 unfiltered images of this event, and several thousand images from before and after. This note presents Sloan r′ band light curves obtained from these images. In addition, a color–magnitude diagram (CMD) using AAVSO data is included to infer parameters of the system’s white dwarf (WD). Features in the light curve allow one to classify this outburst as that of classical (or Z And) type. The CMD suggests the temperature and luminosity of the WD increase concurrently to produce the light curve maxima. Light curves either side of the event show wave-like oscillations owing to the orbital period of AG Pegasi’s components, indicating quiescence.

Spectral behavior of the symbiotic nova AG Pegasi observed with IUE and HST

Ultraviolet spectra from the International Ultraviolet Explorer (IUE) and from the Hubble Space Telescope (HST) of the symbiotic novae AG Peg during the period 1978–1996 are analyzed. Some spectra showing the modulations of spectral lines at different times are presented. We determined the reddening from the 2200Å feature, finding that E(B−V)= 0.10 ± 0.02. We studied N IV] at 1486Å, C IV 1550Å, and O III] at 1660Å, produced in the fast wind from the hot white dwarf. The mean wind velocity of the three emission lines is 1300kms−1 (FWHM). The mean wind mass loss rate is ∼6 × 10−7 M⊙yr−1. The mean temperature is ∼6.5 ×105K. The mean ultraviolet luminosity is ∼5 ×1033ergs−1. The modulations of line fluxes in the emitting region at different times are attributed to the variations of density and temperature of the ejected matter as a result of variations in the rate of mass loss.

AG Pegasi – now a classical symbiotic star in outburst?

Optical spectroscopy study of the recent AG Peg outburst observed during the second half of 2015 is presented. Considerable variations of the intensity and the shape of the spectral features as well as the changes of the hot component parameters, caused by the outburst, are discussed and certain similarities between the outburst of AG Peg and the outburst of a classical symbiotic stars are shown. It seems that after the end of the symbiotic nova phase, AG Peg became a member of the classical symbiotic stars group.

High resolution spectra of the symbiotic star AG Pegasi in 2004–2006

Results of the analysis of observational data obtained over a three-year period from the symbiotic star AG Pegasi are presented in this article; the data presented herein were obtained using a Echelle spectrograph located at the Bohyunsan Astronomical Observatory (BOAO) in Korea. The phases of AG Pegasi range from 0.01 to 0.72. The characteristic line profiles of H I, He I, He II, O I, and Fe II, at wavelengths ranging between 3800 Å and 9000 Å, were observed. These line profiles, except for Fe II, were deconvoluted with several Gaussian functions. On the basis of the features of both the lines that evolved during observations in 2004–2006 and the deconvoluted lines, collision of two winds and jet flows could be confirmed, and a wobble motion of the accretion disk has been suggested.

Colliding Winds in Symbiotic Binary Systems. II. Colliding Winds Geometries and Orbital Motion in the Symbiotic Nova AG Pegasi

AG Pegasi has been observed at high angular resolution and sensitivity at the Very Large Array (VLA) at 5 GHz in four epochs between 1984 and 1991. Analysis of the radio visibilities indicate that a mass of 4.0 ± 0.5 × 10-5 M☉ is concentrated in the inner nebula and is moving outward at a velocity of 53 ± 4 km s-1 (D = 600 pc assumed). In order to explain the observed morphology of the inner nebula, a new colliding winds model is derived, which includes the effects of orbital motion (CWo model). Orbital effects cannot be ignored in AG Pegasi since the orbital timescale (2.25 yr; Meinunger 1981) is short compared to the likely timescale of wind collision (symbiotic nova eruption beginning ~1850; Merrill 1959). When these effects are considered, the interaction front between binary stellar winds is wrapped into spiral walls whose density decreases outward with 1/r2. Distinctive geometries are found to arise depending on which wind dominates the interaction, the late-type wind from the symbiotic cool component, or the high-velocity wind from the Application of the CWo model to AG Peg suggests that the observed transient lobe enhancements of the inner nebula arise due to changes in the mass-loss rate from the hot component. Hot component mass-loss rates ranging between 2.1 and 6.0 × 10-8 M☉ yr-1 are derived. The model is also successful in reproducing the radio spectrum of the central unresolved object of the system. A position angle of -15° ± 10° is inferred for the orbital pole as projected on the plane of the sky.

공생형 별 AG PEGASI의 고해상 스펙트럼

We report a high resolution spectrum of AG Pegasi observed at Bohyunsan Optical Astronomy Observatory (BOAO) on October 2, 2004. Some of permitted emission lines, for example H I, He I, He II, Fe II and Ti II were observed in the spectrum of AG Pegasi in 2004. Lines presented in the longer wavelength region than <TEX>$6500{\AA}$</TEX> are identified. And radial velocities for each element are measured. Then we carefully discuss the geometrical feature of AG Pegasi in October 2004.

The nature of ultraviolet spectra of AG Pegasi and other symbiotic stars: locations, origins, and excitation mechanisms of emission lines

A detailed study of ultraviolet spectra of the symbiotic star AG Peg has been undertaken to derive the atomic excitation mechanisms and origin of formation for the lines common in symbiotic systems. More than 600 emission lines are observed in spectra from IUE, HST and FUSE of which 585 are identified. Population mechanisms and origin of formation are given for a majority of those lines. Based on the understanding of the AG Peg spectra IUE data of 19 additional symbiotic stars are investigated and differences and similarities of their spectra are discussed. Fe II fluorescence lines pumped by strong emission lines between 1000 and 2000 angstrom are observed in 13 of these systems. Some of the symbiotic systems belonging to the subclass symbiotic novae have more than 100 Fe II fluorescence lines in the ultraviolet wavelength region. Forbidden lines are detected for 13 of the stars, mostly from highly-ionized spectra such as Ar V, Ne V and Mg V. Further, [Mg VI] and [Mg VII] lines are observed in a symbiotic star (AG Dra) for the first time. Five of the symbiotic stars have broad white-dwarf wind profiles (FWHM > 400 km s(-1)) for a few lines in their spectra. The stars with no such broad lines can be divided into two similarly sized groups, one where all lines have FWHM less than 70 km s(-1) and the other where one, a few or all of the broad (FWHM > 400 km s(-1)) lines of AG Peg have an enhanced broad wing (110-140 km s(-1)). (Less)

An analysis of the emission line spectra of AG Pegasi between phases 7.34 and 9.44

The UV and optical spectra from the HST Faint Object Spectrograph and from the Fast Spectrograph for the Tillinghast Telescope (FAST) spectrograph at the Whipple Observatory reported by Kenyon, Proga & Keyes at different phases are analysed, leading to new results about the configuration of AG Pegasi. The FAST spectra contain both Ha and Hβ lines, whose ratio changes with phase indicating that different nebulae contribute to each spectrum. In particular, the spectrum emitted from one of the nebulae must be collision dominated, in order to justify the relatively-high Hα/Hβ. Consistent modelling by the code SUMA, which accounts for the coupled effect of the photoionization from the hot star and the shock, shows the important role of at least three nebulae: (1) the nebula between the stars, downstream of the shock created by collision of the winds, which propagates in reverse towards the white dwarf; (2) the nebula downstream of the shock expanding in the outskirts of the giant atmosphere; and (3) the shock propagating outwards from the binary system, which is not reached by the hot source radiation. The relative contributions of the three nebulae to the final spectra at different phases are calculated for all the lines. It is found that the contribution to the different lines changes with the viewing angle of the system. The relatively-low pre-shock density and magnetic field adopted in the modelling of the expanding shocks reveal that they are merging with the ISM, thus explaining the slow decline of AG Pegasi.

The Continuing Slow Decline of AG Pegasi

We analyze optical and ultraviolet observations of the symbiotic binary AG Pegasi acquired during 1992–1997. The bolometric luminosity of the hot component declined by a factor of 2–3 from 1980–1985 to 1997. Since 1992, the effective temperature of the hot component may have declined by 10%–20%, but this decline is comparable to the measurement errors. Optical observations of Hβ and He I emission show a clear illumination effect, where high-energy photons from the hot component ionize the outer atmosphere of the red giant. Simple illumination models generally account for the magnitude of the optical and ultraviolet emission-line fluxes. High-ionization emission lines—[Ne V], [Mg V], and [Fe VII]—suggest mechanical heating in the outer portions of the photoionized red giant wind. This emission probably originates in a low-density region ~30–300 AU from the central binary.

The Evolving Structure of AG Pegasi, Emerging from the Interpretation of the Emission Spectra at Different Phases

A model of AG Peg is presented, focusing on the physical conditions in the emission nebulae. The model accounts in a consistent way for photoionization by the star and ionization by shocks. The SUMA code is used in the calculations of the spectra. We show that two regions contribute to the UV and optical line spectra. The broad lines are emitted from photoionized gas surrounding the hot star, while narrow lines are emitted by shocked gas—which is also reached by the ionizing flux from the hot star—near the red giant. At an early phase ( = 2.33) the emitting region is between the stars but already very close to the red giant. The nebula surrounding the hot star is not exactly symmetric. At a later phase ( = 7.05) the shock created by head-on-back collision of the winds propagates outward and slightly accelerates throughout the giant atmosphere characterized by a decreasing slope of the density. The strong shock caused by head-on collision of the winds from the two stars cannot be recognized in the UV-optical line spectrum. The spectral energy distribution of the continuum is well fitted by blackbody emissions from the stars. The fit of the optical-UV observed continuum at the early phase presents some problems connected with the reddening correction of the data. On the other hand, the fit of the data at a later phase is consistent with the parameters deduced by the modeling of the system. The far-IR data are well fitted by reradiation by dust, which could not be recognized without modeling, and indicate a dust-to-gas ratio similar to the Galactic one.

Evolution of the symbiotic binary system AG Pegasi - The slowest classical nova eruption ever recorded

We present an analysis of new and existing photometric and spectroscopic observations of the ongoing eruption in the symbiotic star AG Pegasi, showing that this binary has evolved considerably since the turn of the century. Recent dramatic changes in both the UV continuum and the wind from the hot component allow a more detailed analysis than in previous papers. AG Peg is composed of a normal M3 giant and a hot, compact star embedded in a dense, ionized nebula. The hot component powers the activity observed in this system, including a dense wind and a photoionized region within the outer atmosphere of the red giant. The hot component contracted in radius at roughly constant luminosity from 1850 to 1985. Its bolometric luminosity declined by a factor of about 4 during the past 5 yr. Both the mass loss rate from the hot component and the emission activity decreased in step with the hot component's total luminosity, while photospheric radiation from the red giant companion remained essentially constant.

A multi-frequency study of symbiotic stars - II. Submillimetre/millimetre observations

It has been suggested that free-free emission from symbiotic stars should become optically thin at submillimetre and millimetre wavelengths. 0.45-2.0 mm continuum measurements are presentred for a selected sample of symbiotic stars, H2-38, HM Sagittae, V1016 Cygni, AG Pegasi and R Aquarii. These observations clearly confirm that the D-type symbiotic stars HM Sge and V1016 Cyg exhibit a spectral turnover at a wavelength of ∼ 3 cm. The 1.1-2.0 mm spectral indices for these sources agree well with that expected for optically thin free-free emission

AG Pegasi - A multishell radio source

view Abstract Citations (31) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS AG Pegasi: A Multishell Radio Source Kenny, H. T. ; Taylor, A. R. ; Seaquist, E. R. Abstract AG Pegasi is one of about 30 symbiotic stars that have been detected as radio sources. It is a symbiotic nova system that has taken longer than 100 yr to return to quiescence after a major outburst (ca. 1850). We present here radio continuum observations of the system made at nine epochs over a period of 3 yr. These observations include data at four frequencies (1.5, 5, 15, 22 GHz) and from all configurations of the Very Large Array (VLA). The observations reveal that circumstellar gas exists in four distinct radio features: an unresolved central source of diameter less than 0".1, an inner nebular shell with diameter of 2".00±+ 0".06 and mass of 4 × 10-5 Msun, and an intermediate nebula with diameter of ∼20" and mass of ∼10-4 to 10-5 Msun, an intermediate nebula with diameter of ∼1' and a mass of 10-5 to 10-4 Msun. The outer nebulosity is thought to be the remnant of the 1850 outburst. The intermediate nebula appears to represent the preeruption mass loss from the system's M3 giant. Comparison of very high resolution observations, 2.7 yr apart, show that the inner nebular shell is expanding at 60±15 km s-1. It is detached from the central unresolved object and is brightening with time. This nebula may arise from an interaction between preeruption mass loss and a fast diffuse wind which is at present emanating from the compact companion of the M3 giant. The flux of the central unresolved object is found to be variable with time and appears to be periodic in phase with the photometric light curve of the system. Publication: The Astrophysical Journal Pub Date: January 1991 DOI: 10.1086/169590 Bibcode: 1991ApJ...366..549K Keywords: Radio Sources (Astronomy); Stellar Envelopes; Symbiotic Stars; Radio Astronomy; Stellar Models; Very Large Array (Vla); stars: circumstellar shells stars: individual (AG Pegasi) stars: radio radiation; stars: symbiotic; Astrophysics; STARS: CIRCUMSTELLAR SHELLS; STARS: INDIVIDUAL CONSTELLATION NAME: AG PEGASI; STARS: RADIO RADIATION; STARS: SYMBIOTIC full text sources ADS | data products SIMBAD (1)

Analysis of the photometric and spectroscopic period of the symbiotic star AG Pegasi with consequences for the model

Analysis of the photometric and spectroscopic period of the symbiotic star AG Pegasi with consequences for the model

A New Absorption-Line Orbit for the Symbiotic Nova AG Pegasi

AbstractPrecise cross-correlation absorption-line velocities have been derived using high resolution near infrared coude’ spectra for the symbiotic nova AG Pegasi. A revised weighted orbital solution is presented based on both extant photographic and the new absorption-line velocities.

The Photometric Period of AG Pegasi

AbstractThe photometric period of AG Pegasi has been determined to be 813 days on the basis of 165 photoelectrical observations from 1961 to 1986. When splitting the interval different periods were found.

AG Pegasi: The episode of minimum 1985

The profiles of the emission lines of Hα, HeI 5876 and HeII 5411 in the spectrum of AG Peg obtained in the minimum of 1985 are described. The light curves in UBVRI system are presented. No indication for the presence of a magnetic field exceeding the error of observations (200 gauss) has been found.

AG Pegasi: the Episode of Minimum 1985

AbstractThe profiles of the emission lines of Hα, HeI 587 6 and HeII 5411 in the spectrum of AG Peg obtained in the minimum of 1985 are described. The light curves in UBVRI system are presented. No indication for the presence of a magnetic field exceeding the error of observations (200 gauss) has been found.

A search for magnetic fields in the symbiotic and VV Cephei variables

The McDonald Observatory's 2.7 m photoelectric analyzer was used to examine five symbiotic and VV Cephei variables for the presence of coherent longitudinal fields. Repeated observations of magnetic Ap stars indicates an absolute sensitivity of +- 100--200 gauss. To this level, no new evidence is found supporting the reported kilogauss fields on the quiescent symbiotics AG Pegasi and EG Andromedae, nor for the VV Cephei stars VV Cephei and WY Geminorum, contrary to extant photographic determinations. Observations of CH Cygni following its 1977 eruption also yielded null results. The lack of significant line broadening correlated with effective z-values further rules out the presence of large transverse components.