Spectroscopic and photometric analysis of the early-type spectroscopic binary HD 161853 in the centre of an H ii region
Key Words.:stars: binaries: spectroscopic – stars: early-type – stars: individual: HD 161853
Aims:We study the O-type star HD 161853, which has been noted as a probable double-lined spectroscopic binary system.
Methods:We secured high-resolution spectra of HD 161853 during the past nine years. We separated the two components in the system and measured their respective radial velocities for the first time.
Results:We confirm that HD 161853 is an 1 Ma old binary system consisting of an O8 V star ( M) and a B1–3 V star ( M) at about 1.3 kpc. From the radial velocity curve, we measure an orbital period = 2.667650.00001 d and an eccentricity = 0.1210.007. Its -band light curve is constant within 0.014 mag and does not display eclipses, from which we impose a maximum orbital inclination deg. HD 161853 is probably associated with an H ii region and a poorly investigated very young open cluster. In addition, we detect a compact emission region at 50 arcsec to HD 161853 in 22m-WISE and 24m-Spitzer images, which may be identified as a dust wave piled up by the radiation pressure of the massive binary system.
HD 161853 (=CPD –31 4999; RA (J2000)=17:49:16.6, Dec (J2000)=–31:15:18; =358.4248, =–1.8767; mag) is an O-type star that has been considered in the literature as either a massive or a post-AGB star. It was first included in the Henry Draper Catalogue (Cannon & Pickering, 1924) and classified as type B3. Subsequently, Crampton (1971, 1972), who also reported radial velocity variations, and Walborn (1973) classified it as O7.5 and O8 V((n)), respectively.
New observations of HD 161853 in the mid- and far-infrared led to confusion about its true nature. For instance, Ratag et al. (1990) proposed it to be a planetary nebula candidate because its radio emission at 4.8 GHz and colours were typical of such objects. Furthermore, Parthasarathy (1993) considered it a post-AGB star rapidly evolving toward a young planetary nebula nucleus. Although Szczerba et al. (2007) disqualified it as a post-AGB star because of its high effective temperature of more than 35 000 K, they did not clarify its actual nature, and more works continued considering HD 161853 as a low-to-intermediate-mass star (e.g. Cerrigone et al., 2011).
HD 161853 is located at the centre of an H ii region associated with a CO molecular cloud (Blitz et al., 1982) at a kinematic distance of 1.50.2 kpc (Hou & Han, 2014). There has also been some confusion because the region was catalogued by Sharpless (1953, 1959) with different running numbers (Sh 1-17 and Sh 2-15). In addition, the H ii region is not unequivocally identified in the SIMBAD database and has many unrelated entries, such as RCW 134 (Rodgers et al., 1960), W 25 (Westerhout, 1958), and G358.464–01.897 (Anderson et al., 2014). The nebula was also identified at H by Gum (1955) as Gum 69 (in the SIMBAD database, Gum 69 is used as an alternative identifier of the star HD 161853).
In the field of HD 161853 there are also other objects, suggesting that the area is a young region that deserves further studies. For example, there are several reported young star candidates (Robitaille et al., 2008), and the open cluster Dutra-Bica 51 (Dutra & Bica, 2000). Dias et al. (2002) determined a distance of 1.3 kpc, an apparent diameter of 2.2 arcmin, and an age of only 1 Ma for the cluster. However, de Wit et al. (2004) did not find a stellar overdensity in the field that could be related to a cluster. A supernova remnant, G358.4–01.9, with dimensions 4036 arcmin, whose centre is 1.5 arcmin south of HD 161853, was reported by Reich et al. (1988). However, Gray (1994) ruled out the existence of such a remnant because the region presents a thermal spectral distribution. Finally, there is a faint source at the position of HD 161853, 1RXS J174916.5–311509 (Voges et al., 2000), also detected by (CXOGBS J174916.6–311518; Jonker et al., 2014, Albacete-Colombo, in prep.).
Multiplicity of HD 161853 has been suspected since the publications of Crampton (1971, 1972). More recently, Mello et al. (2012) noted double He i absorption lines that were related to a binary nature of the source, and Sota et al. (2014) identified them as coming from an O8 V(n)z and B-type components, but they did not compute any orbit determination.
In this paper, we present the spectral classifications of the two components of HD 161853 and their respective radial velocity orbits. We also analyse the available images of the field and use the ASAS -band data to constrain the orbital inclination. Finally, we discuss the actual nature of HD 161853. Preliminary results of this work were shown by Putkuri et al. (2014).
This work is based on observations obtained within the intensive spectroscopic campaign named the OWN Survey (Barbá et al., 2010). We employed the REOSC Cassegrain spectrograph in cross-dispersed mode at the Complejo Astronómico El Leoncito (CASLEO) in Argentina, along with FEROS at the 2.2m telescope of ESO/La Silla, and the échelle spectrograph at the 2.5m du Pont telescope of Las Campanas Observatory (LCO), in Chile. Observations were secured between 2006 May and 2013 August. The instrumental configuration, observatory, spectral resolving power, wavelength coverage, and the number of obtained spectra () are summarised in Table 1.
At CASLEO and LCO, we obtained a wavelength calibration lamp (Th-Ar) exposure immediately before
or after each target integration, at the same telescope position.
The spectra were processed and calibrated using standard IRAF
|Échelle, 2.5-m||LCO||40 000||3450–9850||13|
|REOSC, 2.15-m||CASLEO||15 000||3600–6100||26|
|FEROS, 2.2-m||La Silla||46 000||3570–9210||4|
3 Results and discussion
3.1 Radial velocities
Some of our spectra of HD 161853 clearly show two components, as noted by Mello et al. (2012) and Sota et al. (2014). Thus, we employed the method for separating composite spectra as explained in González & Levato (2006). After a few iterations, we obtained the individual spectra of HD 161853 A and B and radial velocities for the 43 observed epochs. Some Balmer lines showed issues due to incorrect normalisation of the échelle orders containing broad lines.
We performed the cross-correlation by using the fxcor task of IRAF. The wavelength regions considered were 5008–5024 Å and 5864–5888 Å, which include the He i and absorption lines present in both components. fxcor provides errors, but as they are smaller than the expected instrumental errors, we adopted a conservative error of 2.5 km s for all measurements. Moreover, the interstellar Na i Å measured in the spectra resulted in a mean of 10 km s with a maximum difference of 2.5 km s among them, and a standard deviation of 1 km s. We did not find any systematic differences among measurements performed with the three spectrographs used in this work. The Heliocentric Julian Days (HJD) and radial velocities (RV) of the two components are shown in Table 2.
|2 453 867.879||25||0.5||0.07||CAS|
|2 453 875.893||33||0.5||0.08||LCO|
|2 454 222.835||63||1.0||0.13||CAS|
|2 454 246.821||58||1.0||0.12||ESO|
|2 454 251.822||106||0.5||1.00||CAS|
|2 454 252.804||84||1.0||0.36||CAS|
|2 454 253.667||216||1.0||0.69||CAS|
|2 454 257.897||94||1.0||0.27||LCO|
|2 454 258.640||5||0.5||0.55||LCO|
|2 454 258.882||136||1.0||0.64||LCO|
|2 454 286.600||32||…||1.0||0.03||CAS|
|2 454 286.753||39||…||1.0||0.09||CAS|
|2 454 312.603||296||1.0||0.78||CAS|
|2 454 316.490||95||1.0||0.24||CAS|
|2 454 316.611||98||1.0||0.28||CAS|
|2 454 339.469||…||1.0||0.85||CAS|
|2 454 339.584||255||1.0||0.90||CAS|
|2 454 340.506||99||1.0||0.24||CAS|
|2 454 340.608||104||1.0||0.28||CAS|
|2 454 608.891||315||1.0||0.85||CAS|
|2 454 643.765||252||1.0||0.92||CAS|
|2 454 659.757||248||1.0||0.92||CAS|
|2 454 953.769||75||1.0||0.13||ESO|
|2 454 955.765||256||1.0||0.88||ESO|
|2 454 956.780||95||1.0||0.26||ESO|
|2 454 960.790||-90||280||1.0||0.76||LCO|
|2 454 961.818||68||1.0||0.15||LCO|
|2 454 961.830||71||1.0||0.15||LCO|
|2 454 963.715||328||1.0||0.86||LCO|
|2 454 963.723||308||1.0||0.86||LCO|
|2 454 963.921||247||1.0||0.93||LCO|
|2 454 964.662||92||1.0||0.21||LCO|
|2 454 964.922||95||1.0||0.31||LCO|
|2 454 984.868||310||1.0||0.79||CAS|
|2 455 846.507||283||1.0||0.78||LCO|
|2 456 521.470||250||1.0||0.80||CAS|
|2 456 521.678||246||1.0||0.88||CAS|
|2 456 522.473||86||1.0||0.18||CAS|
|2 456 522.659||102||1.0||0.25||CAS|
|2 456 523.605||67||1.0||0.60||CAS|
|2 456 523.707||127||1.0||0.64||CAS|
|2 456 524.524||193||1.0||0.95||CAS|
|2 456 524.687||71||1.0||0.01||CAS|
3.2 Separated spectra
We independently analysed and classified the separated spectra following Sota et al. (2011) for the O-type primary, and following Walborn & Fitzpatrick (1990) for the B-type secondary component. We inspected the spectra visually using the mgb code (Maíz Apellániz et al., 2012), which allows the user to compare the unknown spectrum with standard stars of each sub-type.
We determined a spectral type O8 Vz for the primary and B1–3 V for the secondary. The O8 Vz classification agrees with Sota et al. (2014). The B1–3 sub-type is only determined from the intensity relations among the faint He i lines. Other lines useful as primary criteria, such as Mg ii Å and Si ii Å, are very noisy or affected by residuals in the derived spectrum. The two spectra are shown in Fig. 1.
3.3 Orbital solution
The obtained RV measurements of the primary star were used to search for periodicities by means of the Lomb-Scargle algorithm (Scargle, 1982). This algorithm is provided on-line as a NASA Exoplanet Archive service (Akeson et al., 2013). A period of 2.66 days was obtained.
The orbital solution of the SB2 was determined with an
improved version of the original program for the determination of the orbital elements of
spectroscopic binaries (Bertiau & Grobben, 1969), named gbart and
developed by F. Bareilles
|[HJD]||2 456 634.040.02|
|[HJD]||2 456 634.730.02|
The non-zero orbital eccentricity implies that the age of the system should be younger than the expected circularisation time, . This value was found to be Ma using Eq. 5.9 in Zahn (1977) and adopting the mass and radius from Martins et al. (2005) and the tidal torque constant =3.5 (Zahn, 1975). This means that HD 161853 is a young massive system.
Through comparison of the minimum masses in Table 3 with the masses expected from theoretical models shown in Table 4, we estimated possible inclinations of =547 deg (using the O star mass) and =477 deg (using the B star mass range). The theoretical mass ratio obtained adopting a B3 V sub-type for the secondary star agress better with the observations than adopting B1 V.
|Parameter||O8 V||B1 V||B3 V|
|Martins et al. (2005)||Cox (2000)|
3.4 Photometric analysis
The short period of the binary encouraged us to retrieve and analyse the photometry obtained by the All Sky Automated Survey (ASAS, Pojmánski, 2002). The data, although near the saturation limit, appear to be almost constant between 2001 and 2009. Statistics performed over 813 values (six-pixel aperture photometry with quality labelled as A) gave a mean of =7.956 mag, a standard deviation of 0.014 mag, and a range of 0.084 mag between the lowest and highest values. Thus, the lack of eclipses can also be used to constrain the highest orbital inclination. We fitted the spectroscopic and photometric data with a Wilson-Devinney model (Wilson & Devinney, 1971) by means of the phoebe code (Prša & Zwitter, 2005) and adopting the theoretical stellar parameters shown in Table 4. We determined a highest orbital inclination of =54 deg, and hence the masses should be greater than 22 M and 7.2 M for the O8 V and B1–3 V stars, respectively. We note the agreement between the value of the inclination angle from photometry and the one from the comparison between the minimum masses and those expected from the theoretical models.
The available multi-band photometry permits analysing the interstellar extinction and determination of the spectrophotometric distance to HD 161853. We applied the chorizos code (Maíz-Apellániz, 2004) to the (Sota et al., 2008), Tycho-2 (Høg et al., 2000), and (2MASS; Skrutskie et al., 2006) data. We obtained mag, and mag, which results in a distance modulus =6.242- mag. Adopting mag for the O8 Vz star (Walborn, 1972; Martins et al., 2005) and that the effect of the flux of the B1–3 V star is smaller than the uncertainty (0.45 mag), we derived a distance of kpc, which is consistent with the kinematic distance to the CO molecular cloud (Hou & Han, 2014) and to the Dutra-Bica 51 open cluster (Dias et al., 2002). This determination supports the HD 161853 membership in a star-forming region that should be studied with further tailored observations.
The mid-infrared images of WISE (22 m; Wright et al., 2010) and Spitzer/MIPS (24 m; Rieke et al., 2004) reveal a compact emission region 50 arcsec north-west from HD 161853 (see Fig. 3). This feature is not detected in the shorter wavelength bands of Spitzer and WISE. It may be a dust wave, similar to the arc-like cloud near Ori noted by Caballero et al. (2008). Ochsendorf et al. (2014) explained these regions as the result of surrounding dust piled up by the radiation pressure of the massive star.
The O-type star HD 161853 is an SB2 system with massive components. We separated the composite spectrum with a separation method and classified the primary star as O8 Vz and the secondary as B1–3 V. We determined individual RVs for both components and derived the orbital parameters, obtaining a period of 2.66765 days and an eccentricity of 0.121. We also calculated minimum masses of 11.4 M and 3.8 M for the O- and B-type stars, respectively.
We analysed the photometry of this star available in the ASAS database. The photometry was useful to constrain the orbital inclination to a value lower than 54 deg and hence increasing the minimum masses to 22 and 7.2 M.
The minimum masses determined for the stars and the eccentricity are the most direct and reliable proof that the O-type component is a massive young star and not a post-AGB object. It is located in a very young region in the centre of an H ii region, consistently with the results of the binary analysis. We have identified a compact emission region 50 arcsec north-west from our star. The mid-infrared flux from this dust wave caused HD 161853 to be erroneously considered as a post-AGB star for almost two decades.
Acknowledgements.We thank the referee José A. Caballero for careful reading of our paper and useful suggestions that improved the work. We thank the directors and staffs of CASLEO, LCO, and ESO/La Silla for the use of their facilities and their kind hospitality during the observing runs. CASLEO is operated under agreement between CONICET and the Universities of La Plata, Córdoba and San Juan, Argentina. The Échelle Liège Spectrograph was jointly built by REOSC and Liège Observatory and is on long-term loan from the latter. RHB acknowledges FONDECYT Project No. 1140076. JIA aknowledges financial support from FONDECYT Project No. 11121550. This research has made use of NASA’s Astrophysics Data System, the SIMBAD database operated at CDS, Strasbourg, France, and the Aladin interactive sky atlas developed at CDS.
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