A Close Encounter of the Massive Kind?
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MNRAS 000,1{8 (2016) Preprint 10 September 2021 Compiled using MNRAS LATEX style file v3.0 A close encounter of the massive kind? J. Ma´ız Apell´aniz,1y H. Sana,2 R. H. Barb´a,3 J.-B. Le Bouquin,4 and R. C. Gamen5 1Centro de Astrobiolog´ıa. CSIC-INTA. Campus ESAC. Camino bajo del castillo s/n. E-28 692 Villanueva de la Ca~nada. Madrid. Spain. 2Institute of Astronomy. KU Leuven. Celestijnenlaan 200 D. B-3001 Leuven. Belgium. 3Departamento de F´ısica y Astronom´ıa.Universidad de La Serena. Av. Cisternas 1200 Norte. La Serena. Chile. 4Institut de Plan´etologie et d'Astrophysique de Grenoble (IPAG) UMR 5274. F-38 041. Grenoble. France. 5Instituto de Astrof´ısica de La Plata (CONICET, UNLP). Paseo del Bosque s/n. 1900 La Plata. Argentina. Accepted 2016 October 7. Received 2016 September 7; in original form 2016 August 16 ABSTRACT We have used (a) HST ACS imaging and STIS spectroscopy, (b) ground-based PIO- NIER/VLT long-baseline interferometry, and (c) ground-based spectroscopy from dif- ferent instruments to study the orbit of the extreme multiple system HD 93 129 Aa,Ab, which is composed of (at least) two very massive stars in a long-period orbit with e > 0:92 that will pass through periastron in 2017/2018. In several ways, the system is an η Car precursor. Around the time of periastron passage the two very strong winds will collide and generate an outburst of non-thermal hard X-ray emission with- out precedent in an O+O binary since astronomers have been able to observe above Earth's atmosphere. A coordinated multiwavelength monitoring in the next two years will enable a breakthrough understanding of the wind interactions in such extreme close encounters. Furthermore, we have found evidence that HD 93 129 Aa may be a binary system itself. In that case, we could witness a three-body interaction that may yield a runaway star or a stellar collision close to or shortly after the periastron passage. Either of those outcomes would be unprecedented, as they are predicted to be low-frequency events in the Milky Way. Key words: ephemerides { binaries: visual { stars: individual: HD 93 129 Aa,Ab { stars: kinematics and dynamics { stars: winds, outflows { X-rays: stars 1 INTRODUCTION sive young cluster Trumpler 14 in the Carina Nebula, has long been recognized as one of the hottest and most mas- Massive stars love company, with the vast majority of sive stellar systems in the Galaxy (Walborn 1971). It is the them (if not all) being born in multiple systems of two or prototype of the earliest O spectral subtype of supergiants more stars (Mason et al. 1998; Sana et al. 2012; Sota et al. (O2 If*) and its only example known in the Galaxy (Walborn 2014; Sana et al. 2014). Many of those systems are in low- et al. 2002; Sota et al. 2014). Nelan et al.(2004) resolved it eccentricity, short-period (1 day to a few months) orbits with the Fine Guidance Sensor (FGS) on board the Hub- that lead to slow, prolonged interactions that play an impor- ble Space Telescope (HST) as an astrometric binary (Aa,Ab tant role in their evolution (Pols 1994). Recent studies have components). Ma´ız Apell´aniz et al.(2008) detected a relative discovered a population of eccentric systems with periods astrometric motion between the two components, with the arXiv:1609.08521v2 [astro-ph.SR] 7 Oct 2016 around 1 year (Sim´on-D´ıaz et al. 2015; Ma´ız Apell´aniz et al. separation in the plane of the sky between them decreasing 2015), of which the most extraordinary case is η Car, a high- in a nearly inward direction, and suggested that the system eccentricity (e = 0:9) binary in a 5.54-year orbit (Damineli followed a highly eccentric and/or inclined orbit. The speed 1996; Damineli et al. 2000) where the periastron passages of the inward motion increased in subsequent years, lending have led to violent events that have changed the fate of the credence to the high eccentricity option (Sana et al. 2014; system in short time scales (Smith 2011). Benagl´ıa et al. 2015). The previously published data show a HD 93 129 A, the central system of the compact mas- decrease from a distance of 66 milliarcseconds (mas) in 1996 to 28 mas in 2012 (2.4 mas/a on average). ? Based on observations obtained with the Hubble Space Tele- scope under GO programs 10 205, 10 602, and 10 898 and at the European Southern Observatory under programs 094.C-0397, 506.D-0495, and 297.D-5038. y E-mail: [email protected] c 2016 The Authors 2 J. Ma´ızApell´anizet al. 2 METHODS telescope (La Silla Observatory, Chile) and the Echelle´ spectrograph attached to the du Pont 2.5 m (Las Campanas 2.1 Astrometric data Observatory, Chile). The data were obtained within the We measured the relative astrometric orbit of framework of the OWN Survey (Barb´aet al. 2010). The slit 00 00 HD 93 129 Aa,Ab using positions from: widths used in those observations (2 in FEROS, and 1 in Echelle)´ do not allow us to spatially resolve the Aa and • One previously published epoch from FGS/HST Ab components. They provide nominal resolving powers of (Ma´ızApell´aniz et al. 2008; Benagl´ıa et al. 2015). 48 000 and 45 000, respectively. The data were processed • Two epochs from the High Resolution Camera (HRC) using the FEROS Data Reduction System implemented of the Advanced Camera for Surveys (ACS) onboard HST within ESO-MIDAS software, and the ´echelle package (Ma´ızApell´aniz et al. 2008). in IRAF. Additional processing such as rectification and • Five epochs from the PIONIER four-beam combiner at telluric corrections were performed using IRAF. The final the Very Large Telescope and Interferometer (VLTI) at the signal-to-noise ratio of the one-dimensional extracted European Southern Observatory (ESO) (Le Bouquin et al. spectra depends on the wavelength and the observing night, 2011; Sana et al. 2014). Of those, only the first one had been varying between 100 and 400. We have measured the radial published before. velocity, equivalent width, and full width at half maximum for a number of absorption and emission lines in the spectra The measured values are listed in Table1. in order to determine the behavior of those parameters with The HRC data (from HST GO programs 10 205, 10 602, time. and 10 898) were processed using a crowded-field photome- UVES/VLT observations were obtained on the night of try package created by the lead author that works on the June 27, 2016 with the DIC2 436+760 and 346 + 820 setups, geometrically distorted (not on the drizzled) exposures (An- yielding a continuous wavelength coverage from 305 nm to derson & King 2004). Three different HRC filters (F330W, 498 nm and from 570 nm to 945 nm. Two back-to-back ex- F435W, and F850LP) were used. The excellent quality of the posures of 150 s were obtained in each setup. An entrance double PSF fit can be appreciated in Fig.1. The existence slit of 000: 6 was used for both arms, yielding a nominal spec- of seven unsaturated bright stars belonging to Trumpler 14 tral resolving power of 60 000. The data were processed in in the HRC field allowed us to measure not only relative the standard way using the ESO CPL pipeline v5.7.0 un- motions (in the plane of the sky) but also absolute motions der the REFLEX environment and calibration frames ob- by assuming that the rest of the stars remained station- tained within 12 h of the scientific observations. The ex- ary during the two-year span between the two HRC epochs tracted 1-dimensional spectra were rectified by fitting a low (2004.589 and 2006.586). The F435W and F850LP filters order polymial through the continuum and show a signal- were used for the absolute proper motion measurement. to-noise ratio per resolution element of about 400. To com- The new VLTI/PIONIER observations were obtained plement our time series, archival UVES observations of with the auxiliary telescopes in configuration A0-G1-J2-J3, HD 93 129 Aa,Ab from April 2013 and April 2015 were fur- which provides a maximum baseline of 132 m, correspond- ther retrieved from the ESO archive data base and processed ing to a spatial resolution element in the H-band of 2.5 mas. in a similar way. The observations were processed following the procedure de- scribed in Sana et al.(2014) and references therein. The HD 93 129 Aa,Ab binary was clearly resolved at each epoch. A binary model with three parameters (separation, position 3 RESULTS angle and brightness ratio) was fitted to the visibilities and 3.1 Relative astrometric orbit closure phases in order to obtain sub-milliarcsec precision relative astrometry of the two components of the system. Since only a fraction of the relative astrometric orbit is available and only seven epochs have been obtained so far (Table1), it was not possible to derive precise values for all 2.2 Spectroscopic data seven independent parameters that need to be determined, which we chose to be (among the different possible combi- We observed HD 93 129 Aa,Ab with the Space Tele- nations): [1] orbital period P , [2] periastron epoch T0, [3] scope Imaging Spectrograph (STIS) onboard HST using the eccentricity e, [4] periastron distance d, [5] inclination i, [6] G430M and G750M gratings in 2010.264 aligning the slit ascending node longitude Ω, and [7] the difference between with the position angle of the system at that time, when the the periastron argument and the ascending node longitude pair was separated by 36 mas, to obtain individually resolved ! − Ω.