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A Reassessment of the Kinematics of Pv Cephei

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A Reassessment of the Kinematics of Pv Cephei Revista Mexicana de Astronom´ıa y Astrof´ısica, 46, 375–383 (2010) A REASSESSMENT OF THE KINEMATICS OF PV CEPHEI BASED ON ACCURATE PROPER MOTION MEASUREMENTS L. Loinard,1 L. F. Rodr´ıguez,1 L. G´omez,2 J. Cant´o,3 A. C. Raga,4 A. A. Goodman,5 and H. G. Arce6 Received 2010 June 15; accepted 2010 August 2 RESUMEN Presentamos dos observaciones de la estrella de pre-secuencia principal PV Cephei tomadas con el Very Large Array con una separaci´on de 10.5 a˜nos. Estos datos implican que los movimientos propios de esta estrella son µα cos δ = +10.9 ± −1 −1 3.0 mas a˜no ; µδ = +0.2 ± 1.8 mas a˜no , muy similares a los ya conocidos para HD 200775, la estrella B2Ve que domina la iluminaci´on de la cercana nebulosa de reflexi´on NGC 7023. Este resultado sugiere que PV Cephei no es una estrella desbocada con movimientos r´apidos, como se hab´ıa sugerido en anteriores estudios. La alta velocidad de PV Cep se hab´ıa inferido de los desplazamientos sistem´aticos hacia el este de los bisectores de pares sucesivos de objetos Herbig Haro a lo largo de su chorro. Estos desplazamientos sistem´aticos podr´ıan, en realidad, deberse a una asimetr´ıa en los mecanismos de eyecci´on, o en la distribuci´on del material circunestelar. ABSTRACT We present two Very Large Array observations of the pre-main-sequence star PV Cephei, taken with a separation of 10.5 years. These data show that the proper −1 −1 motions of this star are µα cos δ = +10.9 ± 3.0 mas yr ; µδ = +0.2 ± 1.8 mas yr , very similar to those –previously known– of HD 200775, the B2Ve star that dom- inates the illumination of the nearby reflection nebula NGC 7023. This result suggests that PV Cephei is not a rapidly moving runaway star as suggested by previous studies. The large velocity of PV Cephei had been inferred from the sys- tematic eastward displacement of the bisectors of successive pairs of Herbig Haro knots along its flow. These systematic shifts might instead result from an intrinsic asymmetry in the ejection mechanisms, or in the distribution of the circumstellar material. Key Words: astrometry — ISM: individual (PV Cep) — ISM: jets and outflows — stars: pre-main sequence 1. INTRODUCTION namically unstable (e.g., Valtonen & Mikkola 1991; © Copyright 2010: Instituto de Astronomía, Universidad Nacional Autónoma México Compact stellar clusters as well as multiple stel- Poveda, Ruiz, & Allen 1967). Indeed, close few body lar systems with three or more members can be dy- encounters occurring within them can lead to the 1 acceleration of one or more of the system members, Centro de Radioastronom´ıa y Astrof´ısica, Universidad which –if the acceleration is sufficient– can escape Nacional Aut´onoma de M´exico, Morelia, Mexico. 2Max-Planck-Institut f¨ur Radioastronomie, Bonn, Ger- their birthplace and become runaway stars. It is im- many (Member of the International Max Planck Research portant to search for direct observational evidence School for Astronomy and Astrophysics at the Universities of these energetic events, because if they occurred of Bonn and Cologne). 3Instituto de Astronom´ıa, Universidad Nacional Aut´o- with sufficient frequency during the earliest stages noma de M´exico, Mexico. of star-formation, they could have a significant im- 4Instituto de Ciencias Nucleares, Universidad Nacional pact on the very outcome of the star-forming process Aut´onoma de M´exico, Mexico. (Reipurth 2000). Arguably the most promising case 5Harvard-Smithsonian Center for Astrophysics, Cam- bridge, USA. so far, is that of the BN/KL region in Orion where 6Yale University, New Haven, USA. three stars (including BN itself) are moving away 375 376 LOINARD ET AL. TABLE 1 CHARACTERISTICS OF PV CEP Spectral type A5 R-band magnitude R = 11.1 +155 Distance d = 430−91 pc Mean position (Equinox J2000, α = 20h45m53.s9550 ± 0.s0014 Epoch 2002.26) δ = 67◦57′38.′′681 ± 0.′′008 at about 30 km s−1 from a common point of origin, suggests that the source used to be somewhat to the which happens to be near the center of the massive east of its current position, and thus, that it is mov- KL outflow (Rodr´ıguez et al. 2005; G´omez et al. ing westward relative to its surroundings. Assuming 2005, 2008; Zapata et al. 2009). The dynamical that the HH knots along the flow are fully detached disruption in that case, appears to have occurred a clumps, GA2004 estimated the velocity of the central mere 500 years ago. source to be 22 km s−1 (see discussion in § 2). Most Goodman & Arce (2004, hereafter GA2004) have remarkably, the velocity vector obtained by GA2004 suggested that the star PV Cephei (hereafter PV points almost exactly away from NGC 7023, as if PV 5 Cep) might be another potential candidate. PV Cep Cep had been ejected from NGC 7023 about 10 yr (Table 1) is a Herbig Ae/Be star (Li et al. 1994) ago (see their Figure 6). Thus, PV Cep might in- that drives a well-studied giant Herbig Haro (HH) deed provide us with an example of a runaway star flow (Reipurth, Bally, & Devine 1997; G´omez, Whit- dynamically ejected from its (still identifiable) par- ney, & Kenyon 1997; Arce & Goodman 2002a,b; ent cluster. Note, however, that the conclusions of GA2004). It is located about 1.◦5 west of the re- GA2004 were based on an analysis of the kinematics flection nebula NGC 7023, and the centroid velocity and morphology of the gas surrounding PV Cep, and of its CO emission is similar to that of the CO emis- not on a direct measurement of the relative proper sion from NGC 7023 itself (Cohen et al. 1981). Be- motion between PV Cep and NGC 7023. In the cause of this shared kinematics, it is very likely that present paper (§§ 3 and 4), we will provide such PV Cep and NGC 7023 are at the same distance a direct measurement, combining published optical from the Sun. The measured trigonometric parallax proper motions of the brightest star in NGC 7023 of HD 200775 (MWC 361, V = 7.4 mag) the B2Ve and new radio measurements of the proper motion star that dominates the illumination of NGC 7023 of PV Cep itself. (Herbig 1960) yields a distance for the entire region +155 2. A RE-ANALYSIS OF THE BISECTOR of 430−91 pc (Perryman et al. 1997). This value is consistent with the somewhat older estimate of RESULTS 500 pc used by GA2004. NGC 7023 is associated Several theoretical models (both analytic and nu- with a compact cluster of young stars that contains merical) have been proposed to explain the curved a number of low luminosity pre-main-sequence ob- morphology presented by some HH jets. Cant´o& jects (Lepine & Rieu 1974). Raga (1995) have developed an analytical model © Copyright 2010: Instituto de Astronomía, Universidad Nacional Autónoma México The suggestion by GA2004 that PV Cep might that treats the problem of a steady jet interacting be a runaway star was based on a careful analysis with a sidewind. More recently, Masciadri & Raga of the kinematics and morphology of the gas sur- (2001) presented numerical simulations of a variable rounding PV Cep. The strongest argument follows jet in a sidewind and identified two different regimes. from a study of the distribution of successive pairs For low-amplitude variability (caused by moderate of HH knots located at nearly equal distances from changes in the jet ejection velocity), small working PV Cep along its jet. The segments joining such suc- surfaces (i.e., HH knots) are created along the jet, cessive pairs of knots are expected to pass through and they travel following the path predicted by the the source driving the flow — if the latter is station- steady jet model of Cant´o& Raga (1995). On the ary. In the case of PV Cep, however, GA2004 found other hand, large working surfaces created by larger that the bisectors of the segments were systemati- changes in the jet ejection velocity eventually be- cally shifted eastward of the source, and increasingly come detached “clumps” that travel on somewhat more so, as older HH pairs were considered. This straighter trajectories. PROPER MOTIONS OF PV CEP 377 2.1. Analytic solution for fully detached clumps In their analysis of the bisectors of successive HH pairs, GA2004 assumed that the ejecta had be- come detached clumps, and integrated the equa- tions of motion numerically to obtain the trajec- tory of the clumps. Recently, however, Cant´oet al. (2008) showed that the idealized case of fully detached clumps has a completely analytic solution, which produces trajectories similar to those obtained in numerical simulation. To examine that analytic solution in the case of PV Cep, we assume (follow- ing GA2004) that PV Cep is moving relative to the ambient gas in a direction exactly perpendicular to that of its jet. The velocity of the star relative to the surroundings is taken to be v∗, and individual clumps of mass M are ejected at a velocity ve (see Figure 1a for a schematic description). The trajec- tory is parametrized by a distance s0 (Cant´oet al. 2008) given by: 4 1/3 3 Mv0 s0 ≡ 4 . (1) Fig. 1. (a) Schematic diagram of a star which ejects a 4 ξρ0c bipolar outflow along the positive x-axis, in a direction perpendicular to the motion of the star with respect to Here, ξ is a constant of integration whose numerical the surrounding environment. In the frame of reference value is ξ = 14 (Cant´oet al.
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