of a probable infrared counterpart at R.A. = 18h29ms, decl. The two known X-ray outbursts of XTE J are separated by ~ At coordinates (J) R.A. = 18h29ms, Decl. = d51′”, this XTE J in the XMM-Newton EPIC pn (J) 18 29 XMM-Newton Detection of the s Pulsar XTE J Authors: Halpern, J. P.; Gotthelf Its position is R.A. = 18h29ms, Decl. = d51’23” (J).
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The Cepheid belongs to the peculiar W Vir group, for which the evolutionary status is virtually unknown.
It is the first single-lined system with a pulsating component analyzed using the method developed by Pilecki et al. We show that the presence of a pulsator makes it possible to derive accurate physical parameters of the stars even if radial velocities can be measured for only one of the components.
We have used four different methods to limit and estimate the physical parameters, eventually obtaining precise results by combining pulsation theory with the spectroscopic and photometric solutions. The Cepheid radius, mass, and temperature are, andrespectively, while its companion has a similar sizebut is more massive and hotter K.
Our best estimate for the p -factor of the Cepheid is. The mass, position on the period—luminosity diagram, and pulsation amplitude indicate that the pulsating component is very similar to the Anomalous Cepheids, although it has a much longer period and is redder in color.
The very unusual combination of the components suggest that the system has passed through a mass-transfer phase in its evolution.
More complicated internal structure would then explain its peculiarity. Type II Cepheids are low-mass pulsating stars that belong to the disc and halo populations Wallerstein They are a much older counterpart of the more massive classical Cepheids—they have periods and amplitudes in a similar range, but are about 1. Compared to classical Cepheids, our knowledge of type II Cepheids is very poor, as it is more qualitative than quantitative.
The result of Gingold’s work was only qualitative though, explaining the difference between these three subgroups with different periods, without an explanation for the relative rate of occurrence or a measurement of the values of basic parameters including the masses of these variables.
The situation is even more complicated, as more recent evolutionary models see Bono et al. It is clearly important to obtain direct measurements of the masses for a sample of type II Cepheids to pinpoint their evolutionary status.
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The best means for such measurements is eclipsing binary systems in which one or both components are pulsating stars.
We have applied this method to eclipsing binaries containing classical Cepheids and obtained very precise masses Pilecki et al. The same eclipsing binary method also enables a direct determination of another important parameter, the projection factor hereafter p -factorwhich is the conversion factor between the observed radial velocity RV and the stellar pulsational velocity.
This methodology has been used to measure the p -factor for three classical Cepheids Pilecki et al. These were del peculiar W Virginis stars hereafter also pWVir. In general these stars also lie dscl the normal sequence for type II Cepheids and are bluer in color. For a statistically significant fraction, eclipses and ellipsoidal modulations were detected.
The authors suggested that all of these stars are members of binary systems. The system was later analyzed by Alcock et al. They noted that the components have similar radii, but that the companion to the Cepheid is much brighter and bluer.
[Iaude] MPEC F98: 2018 FS3 [a=1.89,e=0.49,i=4.4,H=21.4,PHA]
They could not say anything about the absolute physical parameters, however. In this study dwcl present high-resolution spectra and use 08 sophisticated modeling, together with pulsation theory, to obtain a consistent picture of the system and the stars 0098 which it is composed, including the important physical parameters like mass of the Cepheid T2CEP Basic data for the system are given in Table 1while in Figure 1 we present all of the photometric data used in the modeling.
VMC Ripepi et al. Dec, eclipses were observed, so the light curve cannot be used for modeling. The K -band data were used to assist in the 09 of the effective temperature of the component stars.
The spectroscopic data were acquired using del MIKE spectrograph on the 6. All the used light curves and RV measurements are available online. We did not use the MACHO R -band data, as the measurements are taken at the same times and do not bring much new information. A cyclical period change was also detected, corresponding to the orbital motion of the Cepheid, and this was taken into account in the fit.
The light curves were also detrended in the process, i. In most cases these were seasonal variations with a period of about one year. To describe the light-curve shape, the amplitude ratios and phase differences are used, with a series of the following form fitted to the data:. For the K -band the third-order Fourier fit is shown as well.
Fourier decomposition— R 21R 31 parameters. Fourier decomposition— and parameters. This observation, together with other features of the star revealed by the analysis presented below, make its classification even more difficult. We will elaborate more on this in Section 5. Once the pulsational light curves were prepared they were subtracted from the raw data to obtain the eclipsing light curves.
Mass and p-factor of the Type II Cepheid OGLE-LMC-T2CEP in a Binary System – IOPscience
The I -band light curve prepared this way presented in Defl 5 was used to obtain an initial photometric solution in order to have a rough estimation of the most important parameters. This estimation was then used as a starting point in the main analysis. I -band orbital light curve of the system, freed from the pulsations of the Cepheid. Both eclipses are well covered. The large difference in eclipse depths indicates a large difference in the temperatures 09 the component stars.
T2CEP is a single-lined spectroscopic binary SB1 and only velocities of the Cepheid could be extracted from the spectra. Although the presence of the companion was detected in the Balmer lines, velocities measured from these lines were not accurate enough to derive its orbit. However, they scatter about the systemic velocity of the system and are clearly anti-correlated with the orbital velocity of the Cepheid. In several spectra other sources were detected at constant velocities: These additional sources are barely visible in the blue part of the spectrum, but are clearly evident in the red portion of the spectrum.
For one of the spectra the profiles of the additional sources obtained this way are even comparable to the Cepheid profile see Figure 6.
However, a prominent third light is excluded by the analysis of the light curves described in Section 4. As the system is located in a very dense field see Figure 7it is highly probable that light from nearby stars on the sky has entered the slit, thus affecting only the obtained spectra and not the photometry.
Note that the slit for the MIKE spectrograph cannot be set since the instrument is fixed with respect to the Nasmyth platform of the Magellan Clay Dexl. As a result, the position angle of the slit on the sky rotates during any individual observation.
When detected, the additional deccl were included in the profile fit at known fixed velocities, making their influence negligible. The field is very crowded, which enhances the possibility for nearby stars to enter the slit and thus appear in the spectra.
The template was taken from the 0988 of theoretical spectra of Coelho et al. In this wavelength range the Cepheid is the only strong variable peak in the majority of spectra. In some cases, where additional light sources were also visible we have included them in the profile fitting with fixed velocities. The overall effect of these sources on the measured Cepheid velocities is thus negligible.
The measured velocities show cyclical variations, with two dfcl corresponding to the pulsational and orbital motions. The orbital period P orb and the time of the spectroscopic conjunction T 0 were kept fixed at the values taken from the preliminary analysis of the photometry. Higher orders might be necessary if the star happens to have a more complicated RV curve, but this cannot be confirmed with the present data.
After fitting this model, a systematic difference in the residuals was seen with higher velocities at the beginning of the observations and lower at the endsuggesting the influence of a third body with a period of the order of ten thousand days. The amplitude is highly correlated with the assumed period, but this assumption has no impact on the other parameters. All of the aforementioned additional objects detected in the spectra showed constant velocity within the errors, so it is highly unlikely that one of them is the supposed third component of the system, as it would have to be much more massive than the stars in the eclipsing binary together.
The results were used to obtain the final photometric model. In the process the orbital solution was updated using new values for P orb and T 0 for full consistency. The eccentricity was also fixed at 0.
The final model parameters are presented in Table 2 and the orbital and pulsational RV curves are shown in Figures 8 and 9respectively. Measured radial velocities of the Cepheid pointswith the removed pulsations overplotted on the orbital solution.
The pulsational variability range of the Cepheid is the light gray area, while the expected RV for the companion velocity is dark gray. Detected additional sources with constant velocities are shown as dotted lines. Pulsational radial velocity recl points and the radius variation solid lines of the Cepheid over one pulsation cycle.
To obtain this RV curve the orbital motion was removed from the measured radial velocities. The full range of isdepending on the assumed p -factor 1.