The latest survey of stellar bow shocks (Peri et al. 2012) lists 28
candidates detected at IR wavelengths, associated with massive, early-type
stars up to 3 kpc, along with the geometrical parameters of the structures
found. I present here some considerations on the energetics involved, after the
estimation of stellar wind power, infrared flux, stellar bolometric luminosity
and radio flux limits for each source. The best candidates for relativistic
particle acceleration are highlighted.Comment: Accepted for the Boletin de la Asociacion Argentina de Astronomia
  201

Benaglia, Paula

English

arXiv

The latest survey of stellar bow shocks (Peri et al. 2012, A&A, 538, 108) lists 28 candidates detected at IR wavelengths, associated with massive, early-type stars up to 3 kpc, along with the geometrical parameters of the structures found. I present here some considerations on the energetics involved, after the estimation of stellar wind power, infrared flux, stellar bolometric luminosity and radio flux limits for each source. The best candidates for relativistic particle acceleration are highlighted.Fil: Benaglia, Paula. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaReunión Anual de la Asociación Argentina de AstronomíaMar del PlataArgentinaInstituto Argentino de Radioastronomí

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Asociación Argentina de Astronomı́aBAAA, Vol. 55, 2012A.C. Rovero, G.E. Romero, L. Pellizza & M. Lares, eds.PRESENTACIÓN MURALEnergetics of nearby stellar bow shocksPaula Benaglia1,2(1) Instituto Argentino de Radioastronomı́a (CCT-La Plata, CONICET)(2) Facultad de Ciencias Astronómicas y Geof́ısicas (FCAG, UNLP)Abstract. The latest survey of stellar bow shocks (Peri et al. 2012,A&A, 538, 108) lists 28 candidates detected at IR wavelengths, associatedwith massive, early-type stars up to 3 kpc, along with the geometricalparameters of the structures found. I present here some considerations onthe energetics involved, after the estimation of stellar wind power, infraredflux, stellar bolometric luminosity and radio flux limits for each source.The best candidates for relativistic particle acceleration are highlighted.Resumen. Se consideran los candidatos a bowshocks listados en el rel-evamiento E-BOSS.v1 (Peri et al. 2012, A&A, 538, 108). Tomando comobase los datos alĺı publicados, se calcula la luminosidad del viento, la lu-minosidad bolometrica de la estrella, la luminosidad correspondiente alflujo WISE medido, y la luminosidad a 1.4 GHz. Se discute la posibilidadde que alguno de los candidatos a bowshocks sea un sitio de aceleración depart́ıculas a velocidades relativistas, con la consiguiente eventual emisióna altas enerǵıas.1. IntroductionRunaway stars move with a speed larger than that of the average of the sur-rounding media, and tend to sweep the material found in the direction of motion.Ideally, the piled-up matter resembles a bow shock, larger for stars with strongwinds. The first systematic search for bow shocks was carried out using the 1988IRAS database (see Noriega-Crespo et al. 1997 and references therein). The au-thors listed ∼ 60 nebulosity sources around early-type stars, some arcmins insize. The advenement of new IR missions like Spitzer or WISE (2011) producedimages at the arcsec resolution, and allowed not only to check former results butto address a much deeper search for stellar bow shocks.With the help of the Tetzlaff et al. (2010) catalogue of runaway stars up to3 kpc, we carried out a study to look for signatures of WISE emission (Wrightet al. 2010) towards all nearby O-B2 stars. The results were compiled in theExtensive stellar BOw Shock Survey, version 1 (E-BOSS.v1, Peri et al. 2012),that lists and describes ∼ 30 bow shocks.Very recently, Benaglia et al. (2010) analyzed the possibility that stellar bowshocks can give rise to high-energy emission, by studying the surroundings ofthe O supergiant BD+43◦3654. We thus seeked for low-frequency radio emissionfrom E-BOSS.v1 candidates, that could be indicative of synchrotron radiation4344 P. Benagliafrom the stellar bowshocks, with the New VLA Sky Survey (Condon et al. 1998,1.4 GHz). We found three new radio sources possible associated with E-BOSScandidates (Peri et al. 2012). Some comments on how common and under whichconditions stellar bow shocks could be high energy emitters are presented here.2. Bow shocks as acceleration sitesThe bow shock of BD+43◦3654 is the prototype of the non-thermal runawaystars. It has been observed with the VLA at two frequencies (L and C band),and the spectral index distribution showed average values α ∼ −0,4 (S ∝ να),characteristic of synchrotron radiation. Benaglia et al. (2010) proposed for thefirst time that a stellar bow shock can host relativistic particles also involved inhigh-energy emission processes, and built a zero-order SED that fits the radioemission and predicts the detectability at shorter wavelengths.The powerful stellar winds of the early-type stars interact with the ambientmedium creating two shocks, separated by a discontinuity: a forward shock witha velocity near the stellar one, and a reverse one fast as the stellar wind (seeBenaglia et al., these Proceedings).Del Valle and Romero (2012) improved the basic model, and applied it tothe case of ζ Oph, concluding that the gamma-ray emission would be weak –ifcompared with other non-thermal emitters- but still detectable by forthcominginstruments like the Cherenkov Telescope Array.The luminosities involved in the phenomenon can be estimated if the mainvariables of the star+bow shock systems are known. The E-BOSS.v1 databasecomprises stellar and bow shock parameters, like the stellar distance d, the windterminal velocity vwind, the stellar mass loss rate Ṁ , the tangential and radialstellar velocities, the stand-off distance R0 and the original ambient density nISMat the position of the star, of each candidate.3. LuminositiesTable 1 lists information on the stellar bow-shock candidates compiled in E-BOSS.v1, related with 28 OB stars. References on spectral types, distances -including error bars- and wind terminal velocities are given in Peri et al. (2012).Stellar velocities were computed from radial and tangential velocities. Distancevalues derived from parallax measurements, and stellar velocities computed fromradial velocity information and proper motions are flagged. The large uncertain-ties in some stellar distances introduce important errors in v∗, R0 and nISM. Thestellar luminosities are from Martins et al. (2005) and Benaglia et al. (2007).The luminosity of the stellar wind is Lwind = 0,5Ṁ(vwind)2, and representsthe available kinetic power. To ensure that the flow is compressible and shockscan develop, the magnetic energy density must be in subequipartition with re-spect to the kinetic energy Lwind. In this case, the value of the magnetic fieldintensity in the flow can be expressed as B2/8π = Lwind/(vwind4πR0) (del Valle& Romero 2012). The authors considered that only a small fraction of the kineticpower available goes into relativistic particles Lrel, and adopted a 10 % factor.Energetics of stellar bow shocks 45The infrared luminosity was estimated by measuring the WISE flux aver-aged over the extension of the bow shock, subtracting the background contribu-tion, and applying the conversion factors (see wise2.ipac.caltech.edu).The bow-shock candidates that correlate with NVSS emission are HIP11891, HIP 38430, HIP 88652 and BD+43◦3654. Their radio luminosity wasderived by measuring the corresponding flux. For the rest of the candidates, anupper limit of 3σ = 3 rms for the NVSS radio flux density was assumed.Both LWISE and LNVSS will also be affected by large distance uncertainties.Distance errors of 50 %, for instance, corresponds to a factor 5 in luminosities.4. DiscussionThe results presented in Table 1 allow to draw some conclusions in regards withthe emission mechanisms acting at the different bow shocks. The Hipparcos stars# 24575, 97796 and 114990 stand out with larger stellar velocities. The first onewas studied at X-rays by López Santiago et al. (2012), who proposed the stellarbow shock as the first X-ray non-thermal emitter.The faster stellar winds will be more efficient accelerators of relativisticparticles, since the acceleration efficiency η ∝ (vshock/c)2 ∼ (vwind/c)2 ≤ 10−4.IC scattering will be favoured wherever the infrared photon field is strong,i.e., for stars with larger LWISE. In the stellar bow-shock scenario, the stellarUV photon field is not relevant to IC, due of the large separation from the star.According to Del Valle & Romero (2012), electron synchrotron losses dom-inate above relativistic Bremsstrahlung. Convection governs proton losses.A crucial factor for high-energy emission detection is the distance to thestar+bow shock system, as the luminosity decays with d2.Finally, the presence of significant radio emission at low frequencies (like1.4 Ghz) is a very strong hint to look for relativistic particles. For those objects,observing campaigns at two or more radio frequencies should be implemented,in full polarization mode if possible, to study the radiation regime. If evidenceof synchrotron emission is found, the conditions mentioned above can help todisentangle the importance of the different contributions to high energy emission.Acknowledgments. P.B. is always grateful to G.E. Romero for useful dis-cussions, and thanks C.S. Peri for comments, and PICT 00848-2007, ANPCyT.ReferenciasBenaglia, P., Vink, J. S., Mart́ı, et al. 2007, A&A, 467, 1265Benaglia, P., Romero, G. E., Mart́ı, et al. 2010, A&A Lett, 517, 10Condon, J. J., Cotton, W. D., Greisen, E. W., et al. 1998, AJ, 115, 1693Del Valle, M. V. & Romero, G. E. 2012, A&A, 543, 56López-Santiago, J. Miceli, M., del Valle, M. V., et al. 2012, ApJ Lett, 757, 6Noriega-Crespo, A., van Buren, D., Dgani, R. 1997, AJ, 113, 780Peri, C. S., Benaglia, P., Brookes, D., et al. 2012, A&A, 538, 108Tetzlaff, N., Neuhausser, R., Hohle, M. M. 2010, MNRAS, 410, 190Wright, E. L., Eisenhardt, P. R. M., Mainzer, A. K., et al. 2010, AJ, 140, 186846P.BenagliaStar Sp.type d nISM R0 v∗ vwind L∗ Lw LWISE LNVSS(kpc) (cm−3) (pc) (km/s) (km/s) log (erg / s)HIP 2036 O9.5III+... 0.76±0,16 130 0.22 16.0 1200 38.7 35.3 36.6 <27.5HIP 2599 B1 Iae 1.50±0,30 0.4 1.27 26.3 1105 39.0 34.7 37.5 <28.0HIP 11891† O5 V((f)) 0.40±0,15 (1) 17 0.12 49.5 2810 38.1 36.4 34.9 30.3HIP 16518 B1 V 0.65±0,16 (1) 0.2 0.13 53.5 500 39.1 32.7 37.2 <27.3HIP 17358 B5 III 0.15±0,10 (1) 600 0.04 (2) 35.2 500 38.0 31.9 35.5 <26.1HIP 22783 O9.5 Ia 1.60±0,30 0.02 4.67 (2) 52.4 1590 39.2 35.3 37.1 <28.1HIP 24575 O9.5 V 0.55±0,07 3 0.06 152.0 1200 38.3 34.7 35.0 <27.2HIP 25923 B0 V 0.90±0,20 (1) 1 0.39 24.2 1000 38.1 34.3 35.8 <27.6HIP 26397 B0.5 V 0.35±0,15 (1) 2 0.10 22.4 750 38.1 33.4 34.6 <26.8HIP 28881 O8 Vn 1.5 : 0.3 1.85 (2) 17.7 2070 38.5 34.6 36.2 <28.1HIP 29276 B1/2 III 0.40±0,03 (1) 0.003 0.23 32.0 600 37.8 32.1 35.6 <26.9HIP 31766 O9.7 Ib 1.40±0,03 0.03 0.82 58.8 1590 39.1 35.9 37.2 <28.0HIP 32067 O5.5V((f))+... 2.10±0,40 0.1 1.85 38.8 2960 39.0 35.6 37.3 <28.4HIP 34536 O6.5V((f))+... 1.30±0,20 0.01 1.5 59.7 2456 38.8 35.6 37.7 <27.9HIP 38430† O6Vn+... 0.9 : (1) 60 0.13 (2) 30.9 2570 38.9 36.2 35.7 31.3HIP 62322 B2.5 V 0.11±0,004 (1) 0.04 0.03 42.2 300 37.9 32.2 33.8 <25.4HIP 72510 O6.5III(n)(f) 0.35±0,18 (1) 0.2 0.15 74.4 2545 39.1 35.7 34.2 <26.8HIP 75095 B1Iab/Ib 0.80±0,50 (1) 40 0.12 28.9 1065 39.0 34.7 35.8 <27.5HIP 77391 O9 I 0.8 : (1) 30 0.23 (2) 24.2 1990 39.2 35.5 37.3 <27.5HIP 78401 B0.2 IVe 0.22±0,02 2 0.39 (2) 38.6 1100 37.8 34.7 36.8 <26.4HIP 81377 O9.5 Vnn 0.22±0,02 1 0.32 28.6 1500 38.3 34.2 35.3 <26.4HIP 82171 B0.5 Ia 0.85±0,12 1 0.17 84.6 1345 39.1 34.7 37.1 <27.6HIP 88652† B0 Ia 0.65±0,30 (1) 2 0.28 31.1 1535 39.0 35.6 35.9 30.1HIP 92865 O8 Vnn 0.35±0,12 (1) 0.003 0.31 (2) 41.2 1755 38.5 33.6 34.6 <26.3HIP 97796 O7.5 Iabf 2.2 : 0.02 3.84 (2) 110.4 1980 39.3 35.8 38.8 <26.8HIP 101186 O9.7 Ia 1.50±0,40 0.1 1.73 35.8 1735 39.1 35.3 38.5 <28.4BD+43 3654 O4 If 1.45± 0.05 0.2 1.48 (2) 67.7 2325 39.5 37.1 35.7 31.2HIP 114990 B0 II 1.4 : 0.05 0.61 (2) 135.7 1400 39.0 35.6 35.2 <28.0†: Potential gamma-ray emitters. They were chosen by IR luminosity, wind terminal velocity and NVSS flux.(1): Stellar distances derived from parallaxes. (2): Stellar velocities derived from radial velocities and proper motions.’:’: errors larger than 50 %.Tabla 1. Stellar, wind and bow-shock luminosities of the 28 bow-shock candidates of Peri et al. 2012.

Energetics of nearby stellar bow shocks

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Energetics of nearby stellar bow shocks

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