Astronomical Institute of the Charles University, Prague. 3 Line profiles ... velocities have been published by Conti & Ebbets (1977), Grigsby et al. (1992), Penny ...
HD 152246 A NEW HIGH-MASS TRIPLE SYSTEM PRELIMINARY RESULTS 1
1
2
2
1
A.Nasseri , R. Chini , P. Mayer , P. Harmanec , T. Dembsky , L.-S. Buda 1
1
2
Astronomisches Institut Bochum, Astronomical Institute of the Charles University, Prague
ABSTRACT: Multi-epoch high-resolution (R ~ 50.000) spectroscopy of the O9 III - IV
1 Introduction
star HD 152246 suggests that it is a hierarchical triple system where an inner pair
Hierarchical triple systems comprise a close binary and a more distant component. They are important for testing theories of star formation and of stellar evolution in the presence of nearby companions. HD 152246 (mV = 7.31) is a poorly investigated member of the Sco OB1 association at a distance of about 1.64 kpc (Sana et al. 2006). Its spectral type was determined as O9 III-IV ((n)) (Walborn 1973). A large range of projected rotational velocities have been published by Conti & Ebbets (1977), Grigsby et al. (1992), Penny (1996) , and Howarth (1997) spanning a range from 72 ≤ V sin i ≤ 280 km/s. Penny (1996) speculated about a possible faint secondary spectrum visible as a blue-ward bump in the cross correlation function. Mason et al. (1998) observed the star with speckle interferometry but could not detect any companion in the angular separation range 0.035” < < 1.5” with m < 3. HD 152246 is part of our monitoring program for stellar multiplicity (Chini et al. 2012) and attracted our attention due to its varying spectral lines.
(Aa Ab) with a circular orbit and a period of 6 days is associated with a third component Ac that displays an eccentric orbit (e = 0.68) and a period of 53 days. The mass ratio for the inner system is probably below 0.2 while the third component has a similar mass as the inner binary. The strengths of various He lines classify Aa and Ac as late O-type stars while Ab is invisible in our spectra and has thus most likely less than 20% of the mass as the other components.
2 Observations
3 Line profiles
We have collected 28 high resolution (R ~ 50.000) Echelle spectra comprising a wavelength range from 3620 to 8530 Å and covering epochs from 2004 - 2013. 20 FEROS spectra were downloaded from the ESO archive; 8 spectra were obtained with BESO at the Universitätssternwarte Bochum (Fig. 1) on a side-hill of Cerro Armazones — the future site of the E-ELT. BESO is a clone of the ESO spectrograph FEROS on La Silla and has been attached to the 1.5 m HexapodTelescope. The S/N ratio for the FEROS spectra is 247 - 487, that for the BESO spectra ranges from 65 to 177, both measured in the neighbourhood of the He I line at 5876 Å. All spectra were reduced with a pipeline, based on a MIDAS package adapted from FEROS.
So far we have analysed limited spectral areas to study the variation of both the radial velocity and the line shape. As an example Fig. 2 shows the profiles of the He I line at 5876 Å. It can be fit by two Gaussians of significantly different FWHM. The same holds for the He II line at 5411 Å (Fig. 3). The absorption at 5404.55 Å and 5418.4 Å associated with the He II 5411 profiles is due to Diffuse Interstellar Bands (Tuairisg et al. 2000).
Fig. 2: Variations of the He I lines at 5876 Å. The equivalent widths of the narrow (green) and broad (blue) components are 0.443 and 0.582, respectively. Left: narrow line left of broad line. Middle: both components are at almost the same velocity. Right: narrow line right of broad line.
Fig. 1: Universitätssternwarte Bochum close to Cerro Armazones. The observatory hosts six telescopes, among them the 1.5 m HexapodTelescope and the 80 cm infrared telescope IRIS.
Fig. 3: Variations of the He II lines at 5411 Å. The equivalent widths of the narrow (green) and broad (blue) components are 0.167 and 0.343, respectively. Left: narrow line left of broad line. Middle: both components are at almost the same velocity. Right: narrow line right of broad line.
4 Orbital solutions
5 KOREL disentangling
6 Spectroscopic analysis
From the line fits shown in Fig. 2 and 3 we produced RV curves. RVs were also measured via a comparison of direct and flipped line profiles. A period analysis showed that the RV of the broader line varies with a period of about 53 days while these of the narrower line were also modulated with a period of 6 days.
The 28 spectra are sufficiently well distributed in the orbital phase to apply the disentangling technique KOREL. In a first step we analysed the wavelength range from 5850—5896 Å with the HeI 5876 line.
To determine Teff, log g and the luminosity ratio of both components we calculated the equivalent widths of several He lines and compared them with synthetic spectra OSTAR (Lanz & Hubeny 2003):
The velocity changes were therefore modelled as a hierarchical triple system with FOTEL (Hadrava 2004a) to compute the orbital solution. The FOTEL orbital elements provided good initial values for the final solution with the spectral-disentangling program KOREL (Hadrava 2004b).
The mass ratio q = M2 /M1 for the inner system Ab/Aa cannot be determined directly due to the absence of spectral lines for Ab. Comparing it with a similar case, HD 165246, (Mayer et al. 2013) where he lines of the secondary also escaped direct detection, we estimate that q is most likely below 0.2. The mass ratio between the inner system and component Ac is 0.96.
We find a period of 6.00474 days for the narrow component and a semiamplitude of the radial-velocity curve of K1 = 39.3 km/s. The epoch of superior conjunction is 54286.11. The broad component has a period of 53.03 days and the corresponding semiamplitudes of the radial-velocity curves are K1+2 = 40.0 km/s and K3 = 41.7 km/s. Likewise, the orbit is widely eccentric with e = 0.68. The periastron epoch is 54297.0.
He I 4922
He II 5411
He I 5876
narrow:
0.116
0.167
0.443
wide:
0.371
0.343
0.582
With these constraints we obtain a plausible temperature of about 32,000 K and log g of 3.5 - 4. As there probably is no large difference in the temperatures of both visible components, the higher brightness of the component with wide lines should be due to smaller log g. We therefore suggest that HD 152246 harbors two late O-type stars, where the component with narrow lines might be of luminosity V while the other one is of IV-III.
References Conti, P.S., Ebbets, D. 1977, ApJ 213, 438
The ratio of periods 53 : 6.0 is similar to the ratio in Tau (33.0 : 3.95), the case considered as the extreme one with such a small value; in all other cases the ratio is considerably larger. Therefore, the system might be at the limit of the dynamical stability of a triple-system .
Grigsby, J.A., Morrison, N.D., Anderson L.S. 1992, ApJS 78, 205 Hadrava, P. 2004a, Publ. Astron. Inst. Acad. Sci. Czech Rep. 92, 1 Hadrava, P. 2004b, Publ. Astron. Inst. Acad. Sci. Czech Rep. 92, 15 Howarth, I.D., Siebert, K.W., Hussain, G.A.J. 1997, MNRAS 284, 265 Lanz, T. , Hubeny, I. 2003, ApJS 146, 417 Mason, B.D. Gies, D.R., Hartkopf, W.I. et al. 1998, AJ 115, 821 Mayer, P., Harmanec, P., Pavlovski, K. 2013, A&A 550, A2 Fig. 4: Disentangled He I 5876 line as obtained from a 2-component KOREL calculation. Component Ac has been shifted by 0.1 in relative flux. Component Ab is not visible at this wavelength.
Penny, L.R. 1996, ApJ 463, 737 Sana, H., Gosset, E., Rauw, G. et al. 2006, A&A 454, 1047 Tuairisg, S.Ó., Cami, J., Foing, B.H. et al. 2000, A&AS 142, 225 Walborn, N.R. 1973, AJ 78, 1067
Acknowledgement. This publication is supported as a project of the Nordrhein-Westfälische Akademie der Wissenschaften und der Künste in the framework of the academy program by the Federal Republic of Germany and the state Nordrhein-Westfalen. The research of PH am PM was supported by the grant P209/10/0715 of the Czech Science Foundation and from the research program MSM0021620860. We acknowledge the continuous support from the Universidad Católica del Norte in Antofagasta, Chile.
