R Coronae Borealis stars in the Galactic Bulge discovered by EROS-2
P. Tisserand, J. B. Marquette, P. R. Wood, E. Lesquoy, J. P. Beaulieu, A. Milsztajn, C. Hamadache, C. Afonso, J. N. Albert, J. Andersen, R. Ansari, E. Aubourg, … 展开作者
P. Tisserand, J. B. Marquette, P. R. Wood, E. Lesquoy, J. P. Beaulieu, A. Milsztajn, C. Hamadache, C. Afonso, J. N. Albert, J. Andersen, R. Ansari, E. Aubourg, P. Bareyre, X. Charlot, C. Coutures, R. Ferlet, P. Fouqué, J. F. Glicenstein, B. Goldman, A. Gould, M. Gros, J. Haissinski, J. de Kat, L. Le Guillou, C. Loup, C. Magneville, E. Maurice, A. Maury, M. Moniez, N. Palanque-Delabrouille, O. Perdereau, Y. Rahal, J. Rich, M. Spiro, A. Vidal-Madjar, S. Zylberajch
Rare types of variable star may give unique insight into short-lived stages of stellar evolution. The systematic monitoring of millions of stars and advanced light curve analysis techniques of microlensing surveys make them ideal for discovering also such rare variable stars. One example is the R Coronae Borealis (RCB) stars, a rare type of evolved carbon-rich supergiant. We have conducted a systematic search of the EROS-2 database for the Galactic catalogue Bulge and spiral arms to find Galactic RCB stars. The light curves of $\sim$100 million stars, monitored for 6.7 years (from July 1996 to February 2003), have been analysed to search for the main signature of RCB stars, large and rapid drops in luminosity. Follow-up spectroscopy has been used to confirm the photometric candidates. We have discovered 14 new RCB stars, all in the direction of the Galactic Bulge, bringing the total number of confirmed Galactic RCB stars to about 51. After reddening correction, the colours and absolute magnitudes of at least 9 of the stars are similar to those of Magellanic RCB stars. This suggests that these stars are in fact located in the Galactic Bulge, making them the first RCB stars discovered in the Bulge. The localisation of the 5 remaining RCBs is more uncertain: 4 are either located behind the Bulge at an estimated maximum distance of 14 kpc or have an unusual thick circumstellar shell; the other is a DY Per RCB which may be located in the Bulge, even if it is fainter than the known Magellanic DY Per. From the small scale height found using the 9 new Bulge RCBs, $61<h^{RCB}_{Bulge}<246$ pc (95% C.L.), we conclude that the RCB stars follow a disk-like distribution inside the Bulge.
Optical high-resolution spectra of the R Coronae Borealis star V CrA at light maximum and during minimum light arediscussed. Abundance analysis confirms previous results showing that V CrA has the composition of the small subclass of R Coronae Borealis (RCB) stars know as `minority' RCBs, i.e., the Si/Fe and S/Fe ratios are 100 times their solar values. A notable novel result for RCBs is the detection of the 1-0 Swan system $^{12}$C$^{13}$C bandhead indicating that $^{13}$C is abundant: spectrum synthesis shows that $^{12}$C/$^{13}$C is about 3 to 4. Absorption line profiles are variable at ma…
In 2001, V2552 Oph (CD -22 12017, HadV98) quickly faded by several magnitudes in a manner typical of the R Coronae Borealis (RCB) stars. Photometry of V2552 Oph obtained over 70 years previous to 2001 shows no indication of variability. Optical spectra of this star subsequently confirmed that V2552 Oph is a member of the hydrogen deficient, carbon-rich RCB class of variables. It resembles the warm (T$_{eff}\sim$ 7000 K) RCB stars such as R Coronae Borealis itself. Other RCB stars, such as XX Cam and Y Mus, have experienced similar periods of inactivity, going decades without significant dust f…
Some of the observational aspects related to the evolutionary status and dust production in R Cor Bor stars are discussed. Recent work regarding the surface abundances, stellar winds and evidence for dust production in these high luminosty hydrogen deficient stars are also reviewed. Possibility of the stellar winds being maintained by surface magnetic fields is also considered.
Neutral fluorine (F I) lines are identified in the optical spectra of several R Coronae Borealis stars (RCBs) at maximum light. These lines provide the first measurement of the fluorine abundance in these stars. Fluorine is enriched in some RCBs by factors of 800 to 8000 relative to its likely initial abundance. The overabundances of fluorine are evidence for the synthesis of fluorine. These results are discussed in the light of the scenario that RCBs are formed by accretion of an He white dwarf by a C-O white dwarf. Sakurai's object (V4334 Sgr), a final He-shell flash product, shows no detect…
Neutral fluorine lines are identified in the optical spectra of several R Coronae Borealis stars (RCBs) at maximum light. These lines provide the first measurement of the fluorine abundance in these stars. Fluorine is enriched in some RCBs by factors of 800 to 8000 relative to its likely initial abundance. The overabundances of fluorine are evidence for the synthesis of fluorine. These results are discussed in the light of the scenario that RCBs are formed by accretion of an He white dwarf by a C-O white dwarf. Sakurai's object (V4334 Sgr), a final He-shell flash product, shows no detectable n…
Two high-resolution optical spectra of the R Coronae Borealis star UW Cen in decline are discussed. A spectrum from mid-1992 when the star had faded by three magnitudes shows just a few differences with the spectrum at maximum light. The ubiquitous sharp emission lines seen in R CrB at a similar drop below maximum light are absent. In contrast, a spectrum from mid-2002 when the star was five magnitudes below maximum light shows an array of sharp emission lines and a collection of broad emission lines. Comparisons are made with spectra of R CrB obtained during the deep 1995-1996 minimum. The ma…
We have obtained high resolution echelle spectroscopy of the recurrent nova T CrB. We find that the surface lithium abundance in T CrB is signifcantly enhanced compared to field M giants, where it is not detectable. We offer possible explanations for this in terms of either a delay in the onset of convection in the giant star, enhanced coronal activity due to star-spots or the enhancement of Li resulting from the nova explosion(s).
The high resolution spectra of a R CrB type star Z UMi are analysed. The atmospheric parameters of Z UMi are estimated: Teff=5250+/-250K and log g=0.5+/-0.3. This places Z UMi among the coolest R CrB stars. The hydrogen deficiency of Z UMi is confirmed. The abundances of other elements resemble those found for the minority group of R CrB stars. We note very low iron abundance, [Fe/H]=-1.85, and an excess of lithium, [Li/Fe]=+1.9.
Abundances of R Coronae Borealis stars (RCBs) and Extreme Helium stars (EHes) are discussed. Recent estimates of the $s$-process elements in hot extreme helium stars show enhancements of lighter $s$-process elements (Y, Zr) relative to heavier $s$-process elements, a characteristic shared by RCB stars. It also suggests that atleast some EHe stars went through an episode of $s$-process element synthesis in their earlier evolution. A majority of RCB stars show a high $^{12}$C/$^{13}$C ratio in their atmospheres. A recent analysis of the spectrum of minority RCB star V CrA, however, shows a lower…
Infrared Space Observatory 3 - 25 $μ$m spectra of the R Coronae Borealis stars V854 Cen, R CrB, and RY Sgr are presented and discussed. Sharp emission features coincident in wavelengths with the well known Unidentified Emission Features are present in the spectrum of V854 Cen but not of R CrB or RY Sgr. Since V854 Cen is not particularly H-poor and has a 1000 times more H than the other stars, the emission features are probably from a carrier containing hydrogen. There is a correspondence between the features and emission from laboratory samples of hydrogenated amorphous carbon. A search for C…
The amplitude of the ellipsoidal variability, the mass function, and the evolutionary limits on the component masses have been used to constrain the binary system parameters. Contrary to all previous studies, our analysis shows that the mass ratio of T CrB q(M_giant/M_hot)=0.6, which implies a low mass binary system, with the stellar masses M_giant=0.7 solar mass, and M_hot=1.2 solar mass. This result strongly supports the thermonuclear runaway model for this recurrent nova, and solves all controversies about the nature of the hot component and the physical causes of its eruptions.
Two sets of high-resolution spectra of R CrB obtained during the 2003 light decline are described. The first set was obtained on the descending branch of the light curve when V ~12.0 and the second one in the recovery phase with V ~7.5. The usual sharp and broad emissions are described and the lines radial velocities measured. C_2 Swan system (0,0) band was found to be in emission for the first set. The other C_2 bands were in absorption. Few CN red system (5,1) band rotational lines and low excitation FeI lines were in absorption. A table with measured radial velocities of various spectral fe…
We have identified five new R Coronae Borealis (RCB) stars in the Galactic bulge using the MACHO Project photometry database, raising the total number of known Galactic RCB stars to about 40. We have obtained spectra to confirm the identifications. The fact that four out of the five newly identified RCB stars are ``cool'' (T(eff) < 6000 K) rather than ``warm'' (T(eff) > 6000 K) suggests that the preponderance of warm RCB stars among the existing sample is a selection bias. These cool RCB stars are redder and fainter than their warm counterparts and may have been missed in surveys done with blu…
EROS 2 (Experience de Recherche d'Objets Sombres) conducted a survey of the SMC between July 1996 and February 2003 in two EROS broad-band colours, V_E and R_E. The photometric data of 4.2 million stars have been searched for behaviour typical of R Coronae Borealis (RCB) candidates such as drastic changes in magnitude. Five objects have been found, four of them being catalogued in the Simbad database as RAW 21, RAW 233, RAW 476, and [MH95] 431 with confirmed carbon-rich atmospheres, characteristic of RCB. From the EROS 2 light curve of RAW 21 and its spectrum reported by Morgan et al. (2003), …
We report the discovery of near-sinusoidal radial velocity variations of the G0V star rhoCrB, with period 39.6 days and amplitude 67 m/s. These variations are consistent with the existence of an orbital companion in a circular orbit. Adopting a mass of 1.0 M(Sun) for the primary, the companion has minimum mass about 1.1 Jupiter masses, and orbital radius about 0.23 AU. Such an orbital radius is too large for tidal circularization of an initially eccentric orbit during the lifetime of the star, and hence we suggest that the low eccentricity is primordial, as would be expected for a planet forme…
A new interpretation for the second peak of T Coronae Borealis (T CrB) outbursts is proposed based on a thermonuclear runaway (TNR) model. The system consists of a very massive white dwarf (WD) with a tilting accretion disk and a lobe-filling red-giant. The first peak of the visual light curve of T CrB outbursts is well reproduced by the TNR model on a WD close to the Chandrasekhar mass ($M_{\rm WD} \gtrsim 1.35 ~M_\odot$), while the second peak is reproduced by the combination of the irradiated M-giant and the irradiated tilting disk. The derived fitting parameters are the WD mass $M_{\rm WD}…
Recently Gatewood, Han & Black presented an analysis of the Hipparcos stellar positions and their own ground-based measurements of rho CrB, suggesting an astrometric orbit of 1.5 mas with an extremely small orbital inclination of 0.5 degrees. This indicates that the planet-candidate secondary might be a late M star. We used the Hipparcos data of rho CrB together with the individual radial velocities of Noyes et al. to independently study the stellar orbit and to assess its statistical significance. Our analysis yielded the same astrometric orbit. However, a permutation test we performed on the…
We examine long-term low-dispersion IUE, SWP and LWP spectroscopy of the R Coronae Borealis (RCB) star V854 Cen, obtained across the deep 1991, 1992-1993 and 1994 declines. We also report the optical light curve for the star in the interval 1987-1998, including multi-color photometry obtained during 1989-1998. Analysis of the UV emission line spectra indicates most lines decay during the deep declines on characteristic timescales comparable to that reported for optical features. Fe, Mg and neutral C lines decay on timescales of typically 50-100 d. Other lines, notably ionized C lines, decay on…
The well-known light minima of stars with the R Coronae Borealis variability are caused by the formation of an additional circumstellar dust shell, the screening shell, inside the permanent shell. Under the assumption of uniform distribution of matter in the circumstellar environment we estimated the optical thickness of the permanent gas-and-dust shell at 0.2-0.7, and its geometrical thickness is no less than 0.4 of its own radius. The wavelength dependence of extinction is close to neutral. From spectral observations of R CrB itself in the 1985 minimum we traced the transformation of the ste…
(Abridged) We present results of XMM-Newton observations of the RS CVn binary $σ^2$ Coronae Borealis. The RGS and EPIC MOS2 spectra were simultaneously fitted with collisional ionization equilibrium plasma models to determine coronal abundances of various elements. Contrary to the solar first ionization potential (FIP) effect in which elements with a low FIP are overabundant in the corona compared to the solar photosphere, and contrary to the ``inverse'' FIP effect observed in several active RS CVn binaries, coronal abundance ratios in $σ^2$ CrB show a complex pattern as supported by similar f…
Using the 1-m telescope at Ritter Observatory, we took 36 observations of zeta^2 Coronae Borealis with a fiber-fed echelle spectrograph. From these observations, zeta^2 CrB was found to be a triple system and a new spectroscopic orbit was calculated. This orbit has two periods, a 1.72357 day period for the inner binary composed of zeta^2 CrB A & B and a 251 day period for the outer binary composed of zeta^2 CrB AB & C. The inner binary is a double-lined spectroscopic binary composed of two B7 V stars. The inner binary's center of mass (zeta^2 CrB AB) describes a long-period single-lined variat…
MS 1603.6+2600 is a high-latitude X-ray binary with a 111 min orbital period, thought to be either an unusual cataclysmic variable or an unusual low-mass X-ray binary. In an ASCA observation in 1997 August, we find a burst, whose light curve suggests a Type I (thermonuclear flash) origin. We also find an orbital X-ray modulation in MS 1603.6+2600, which is likely to be periodic dips, presumably due to azimuthal structure in the accretion disk. Both are consistent with this system being a normal low-mass X-ray binary harboring a neutron star, but at a great distance. We tentatively suggest that…
The 3D Radioastronomical Approach to Doppler tomography has been used to study the H$α$ emission sources in U Coronae Borealis. These 3D tomograms provide greater resolution than the projected 2D version and highlight the jet-like gas flows in the $V_z$ direction transverse to the orbital plane. In this paper, the 3D tomograms are compared at two distinct epochs when U CrB was in the disk-like state (1993 data) and the stream-like state (1994 data). Both states display a prominent emission source, the circumprimary bulge, which is produced when the gas stream strikes the photosphere of the mas…
We present electro-photometric UBV and high-speed U-band flickering observations of the recurrent nova T CrB during a period when its U brightness varies by more than 2 mag. The V band is dominated by the ellipsoidal variability of the red giant, however, the variability of the hot component also causes 0.15 mag variations in V. We define a set of parameters which characterise the flickering. The Fourier spectra of all 27 nights are similar to each other. The power spectral density of the variations has a power law component (f^{-1.46} on average). We do not detect a dependence of the Fourier …
We investigate the evolutionary status of four stars: V348 Sgr, DY Cen and MV Sgr in the Galaxy and HV 2671 in the LMC. These stars have in common random deep declines in visual brightness which are characteristic for R Coronae Borealis (RCB) stars. RCB stars are typically cool, hydrogen deficient supergiants. The four stars studied in this paper are hotter (T$_{\rm eff}$ = 15-20 kK) than the majority of RCB stars (T$_{\rm eff}$ = 5000-7000 K). Although these are commonly grouped together as the \emph{hot RCB stars} they do not necessarily share a common evolutionary history. We present new ob…
Spectra of several R Coronae Borealis (RCB) stars at maximum light were examined for the [C I] 9850 A and 8727 A absorption lines. The 9850 A line is variously blended with a Fe II and CN lines but positive identifications of the [C I] line are made for R CrB and SU Tau. The 8727 A line is detected in the spectrum of the five stars observed in this wavelength region. Carbon abundances are derived from the [C I] lines using the model atmospheres and atmospheric parameters used by Asplund et al. (2000). Although the observed strength of a C I line is constant from cool to hot RCB stars, the stre…
R Coronae Borealis (RCB) stars are hydrogen-deficient, carbon-rich pulsating post-AGB stars that experience massive irregular declines in brightness caused by circumstellar dust formation. The mechanism of dust formation around RCB stars is not well understood. It has been proposed that CO molecules play an important role in cooling the circumstellar gas so that dust may form. We report on a survey for CO in a sample of RCB stars. We obtained H- and K-band spectra including the first and second overtone CO bands for eight RCB stars, the RCB-like star, DY Per and the final-helium-flash star, FG…
We report the discovery of eight new R Coronae Borealis (RCB) stars in the Large Magellanic Cloud (LMC) using the MACHO project photometry database. The discovery of these new stars increases the number of known RCB stars in the LMC to thirteen. We have also discovered four stars similar to the Galactic variable DY Per. These stars decline much more slowly and are cooler than the RCB stars. The absolute luminosities of the Galactic RCB stars are unknown since there is no direct measurement of the distance to any Galactic RCB star. Hence, the importance of the LMC RCB stars. We find a much larg…
We present an updated spectroscopic orbit and a new visual orbit for the double-lined spectroscopic binary σ^2 Coronae Borealis based on radial velocity measurements at the Oak Ridge Observatory in Harvard, Massachusetts and interferometric visibility measurements at the CHARA Array on Mount Wilson. σ^2 CrB is composed of two Sun-like stars of roughly equal mass in a circularized orbit with a period of 1.14 days. The long baselines of the CHARA Array have allowed us to resolve the visual orbit for this pair, the shortest period binary yet resolved interferometrically, enabling us to determine …
We report our search for variability in the H-alpha emission line of the recurrent novae T CrB with time resolution 10-15 minutes. This is comparable with the time scale of the photometric flickering observed in this object. This is the first time that observations of the short time scale variation in emission lines have been made in this object. On two nights (990106 and 990107) we detected statistically significant variability (at 0.99 confidence level) in the line profile of H-alpha. This variability is confined in the central part of the emission line (\pm 100 km/s) although FWZI(H-alpha) …
We present new spectroscopic observations of the He I $λ$10830 line in R Coronae Borealis (RCB) stars which provide the first strong evidence that most, if not all, RCB stars have winds. It has long been suggested that when dust forms around an RCB star, radiation pressure accelerates the dust away from the star, dragging the gas along with it. The new spectra show that nine of the ten stars observed have P-Cygni or asymmetric blue-shifted profiles in the He I $λ$10830 line. In all cases, the He I line indicates a mass outflow - with a range of intensity and velocity. Around the RCB stars, it …
We report the photometric observations of an SU UMa-type dwarf nova VW CrB during two superoutbursts in 2001 and 2003 and a normal outburst in 2003. Superhumps with a period of 0.07287(1) d were observed during the 2003 superoutburst. The change rate of the superhump period was positive. During the normal outburst, there are some hint of modulation up to a 0.2-mag amplitude. However, any periodicity was not found. The recurrence cycles of the normal outburst and the superoutburst, and the distance were estimated to be > ~50 d, 270-500 d, and 690(+230, -170) pc, respectively. These recurrence c…
We analyze H-alpha observations of the recurrent nova T CrB obtained during the last decade. For the first time the H-alpha emission profile is analyzed after subtraction of the red giant contribution. Based on our new radial velocity measurements of the H-alpha emission line we estimate the component masses of T CrB. It is found that the hot component is most likely a massive white dwarf. We estimate the inclination and the component masses to be i~67 deg, Mwd = 1.37 +/-0.13 Msun and Msec=1.12 +/-0.23 Msun, respectively. The radial velocity of the central dip in the H-alpha profile changes ne…
We report precise Doppler measurements of two evolved stars, kappa CrB (HD142091) and HD 167042, obtained at Lick Observatory as part of our search for planets orbiting intermediate-mass subgiants. Periodic variations in the radial velocities of both stars reveal the presence of substellar orbital companions. These two stars are notably massive with stellar masses of 1.80 Msun and 1.64 Msun, indicating that they are former A-type dwarfs that have evolved off of the main sequence and are now K-type subgiants. The planet orbiting kappa CrB has a minimum mass Msini = 1.8 Mjup, eccentricity e = 0.…
We have found that at least seven hydrogen-deficient carbon (HdC) and R Coronae Borealis (RCB) stars, have 16O/18O ratios close to and in some cases less than unity, values that are orders of magnitude lower than measured in other stars (the Solar value is 500). Greatly enhanced 18O is evident in every HdC and RCB we have measured that is cool enough to have detectable CO bands. The three HdC stars measured have 16O/18O < 1, lower values than any of the RCB stars. These discoveries are important clues in determining the evolutionary pathways of HdC and RCB stars, for which two models have been…
We report the discovery of two new R Coronae Borealis (RCB) stars in the Large Magellanic Cloud (LMC) using the MACHO project photometry database. The identification of both stars has been confirmed spectroscopically. One is a cool RCB star (T_eff about 5000 K) characterized by very strong Swan bands of C_2 and violet bands of CN, and weak or absent Balmer lines, G-band and 12C-13C bands. The second star is an example of a hot RCB star of which only 3 were previously known to exist in the Galaxy and none in the LMC. Its spectrum is characterized by several C II lines in emission. Both stars ha…
We present an extended optical spectropolarimetry of R CrB from 1998 January to 2003 September. The polarization was almost constant in the phase of maximum brightness, being consistent with past observations. We detected, however, temporal changes of polarization ($\sim 0.5$ %) in 2001 March and August, which were the first detection of large polarization variability in R CrB near maximum brightness. The amplitude and the position angle of the `transient polarization' were almost constant with wavelength in both two events. There was a difference by about 20 degrees in the position angle betw…
High-resolution optical spectra of R CrB obtained in 2003 March is discussed. The 2003 spectra are compared with the extensive collection of spectra from the 1995-1996 minimum. Spectroscopic features common to the two minima include sharp emission lines of neutral and singly-ionized atoms, broad emission lines including He {\sc i}, [N {\sc ii}] 6583 Å, Na D, and Ca {\sc ii} H & K lines, and blueshifted aborption lines of Na D, and K {\sc i} resonance lines. Prominent differences between the 2003 and 1995-96 spectra are seen. The 2003 profiles of Na D, Ca II H & K are fit by a single Gaussian b…
A set of high-resolution optical spectra of RCrB acquired before, during, and after its 1995-1996 decline is discussed. All of the components reported from earlier declines are seen. This novel dataset provides new information on these components including several aspects not previously seen in declines of RCrB and other RCBs. In the latter category is the discovery that the decline's onset is marked by distortions of absorption lines of high-excitation lines, and quickly followed by emission in these and in low excitation lines. This 'photospheric trigger' implies that dust causing the declin…
We present the results of UBVJHKLM photometry of R CrB spanning the period from 1976 to 2001. Studies of the optical light curve have shown no evidence of any stable harmonics in the variations of the stellar emission. In the L band we found semi-regular oscillations with the two main periods of ~3.3 yr and 11.9 yr and the full amplitude of ~0.8 mag and ~0.6 mag, respectively. The colors of the warm dust shell (resolved by Ohnaka et al. 2001) are found to be remarkably stable in contrast to its brightness. This indicates that the inner radius is a constant, time-independent characteristic of t…
We present new images of the reflection nebula surrounding the R Coronae Borealis Star, UW Cen. This nebula, first detected in 1990, has changed its appearance significantly. At the estimated distance of UW Cen, this nebula is approximately 0.6 ly in radius so the nebula cannot have physically altered in only 8 years. Instead, the morphology of the nebula appears to change as different parts are illuminated by light from the central star modulated by shifting thick dust clouds near its surface. These dust clouds form and dissipate at irregular intervals causing the well-known declines in the R…
We have obtained long-slit far-ultraviolet (1150--1730 A) spectra of the R Coronae Borealis (RCB) stars V854 Cen and RY Sgr, near maximum light and pulsational phase zero, with the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope (HST). The far-UV spectrum of each star shows a photospheric continuum rising steeply toward longer wavelengths, and a prominent emission feature at C II 1335. RY Sgr displays a second, but fainter, emission attributed to Cl I 1351 (which is radiatively fluoresced by C II 1335), but Cl I is weak or absent in V854 Cen. Most surprisingly, the C II e…
R Coronae Borealis variable stars are suspected to sporadically eject optically thick dust clouds causing, when one of them lies on the line-of-sight, a huge brightness decline in visible light. Mid-infrared interferometric observations of RYSgr allowed us to explore the circumstellar regions very close to the central star (~20-40 mas) in order to look for the signature of any heterogeneities. Using the VLTI/MIDI instrument, five dispersed visibility curves were recorded with different projected baselines oriented towards two roughly perpendicular directions. The large spatial frequencies visi…
