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The Star-Forming Region in Orion KL

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The Star-Forming Region in Orion KL Astronomy Letters, Vol. 29, No. 10, 2003, pp. 641–643. Translated from Pis’ma vAstronomicheski˘ı Zhurnal, Vol. 29, No. 10, 2003, pp. 723–726. Original Russian Text Copyright c 2003 by Matveyenko, Zakharin, Diamond, Graham. The Star-Forming Region in Orion KL L. I. Matveyenko1*, K.M.Zakharin1,P.J.Diamond2**, andD. A. Graham 3*** 1Space Research Institute, Profsoyuznaya ul. 84/32, Moscow, 117997 Russia 2Nuffield Radio Observatory, Macclesfield, Cheshire SK11, 9DL, UK 3Max-Planck-Institut fur¨ Radioastronomie, Auf dem Hugel¨ 69, 53121 Bonn, Germany Received May 12, 2003 Abstract—We have studied the fine structure of the active H2O supermaser emission region in Orion KL with an angular resolution of 0.1 mas. We found central features suggestive of a bipolar outflow, bullets, and an envelope which correspond to the earliest stage of low-mass star formation. The ejector is a bright ∼ 17 compact source ≤0.05 AU in size with a brightness temperature Tb = 10 K. The highly collimated bipolar ∼ −1 outflow ∼30 has a velocity vej = 10 km s , a rotation period of ∼0.5 yr, a precession period of ∼10 yr, and a precession angle of ∼33◦. Precession gives rise to a jet in the shape of a conical helix. The envelope amplifies the radio emission from the components by about three orders of magnitude at a velocity v =7.65 km s−1. c 2003 MAIK “Nauka/Interperiodica”. Key words: star formation, accretion disks, maser emission. INTRODUCTION polarized, P ≈ 70%. We interpreted the chain of com- pact components as a thin disk separated into rings Gravitationalinstabilitiesin gas –dust complexes during Keplerian motion seen edge-on (Matveenko give rise to local active zones that contain protostars et al. 1988). This modelassumes the presence of in the phase of gravitationalcontraction. Physical a centralprotostar with a mass of (0.1 –0.5)M,an ∼ conditions and chemicalreactions leadto the forma- inner ring radius Rin = 6 AU, a rotation velocity tion of both simple and complex molecules accom- −1 ∼ vrot =5 km s , and an expansion velocity vexp = panied by intense water-vapor maser emission. We − ∼ 3.8 km s 1. The outer ring radius is R = 16.5 AU investigated the fine structure of the H2O supermaser −1 zone in Orion KL at various epochs by using global and vrot =3km s . The expansion velocity corre- VLBI network observations in 1985 and NRAO VL- sponds to “maser rings” containing sublimed water- BA observations at subsequent epochs. vapor molecules accelerated by radiation pressure and stellar wind. The active region is surrounded by an envelope that amplifies the emission of the THE EPOCH OF ACTIVITY 1979–1987 4 Outbursts of H2O supermaser emission were observed in Orion KL from 1979 to 1987. In Octo- 2 A ber 1985, the maser emission was concentrated in a D 7.8 B ∼ 6.5 C narrow line profile with a width of 42 kHz; the peak 0 flux densities reached Fpeak ∼ 2 MJy (Matveenko 7.8 1981). A 10.8-AU-long chain of five groups of –2 7.8 7.7 6.5 6.6 7.5 7.1 6.4 6.3 compact sources was observed; the sizes of the 8.8 7.9 7.2 6.9 –4 individualsources were ∼ 0.1 AU (Fig. 1). The group 8.0 velocities increased along the chain from 6.45 (west) to 8.75 km s−1 (east). The brightest components had 5 0 –5 –10 −1 16 −1 v =7.7 km s , Tb =5× 10 Kandv =7.9 km s , × 16 Fig. 1. The distribution of maser spots in October 1985. Tb =9 10 K. The maser emission was linearly Relative right ascension and declination (in mas) are along the vertical and horizontal axes, respectively. The *E-mail: [email protected] circle diameters are proportional to the logarithm of the **E-mail: [email protected] intensity of the components. The maximum brightness is *** 17 E-mail: [email protected] Tb ≈ 10 K. 1063-7737/03/2910-0641$24.00 c 2003 MAIK “Nauka/Interperiodica” 642 MATVEYENKO et al. ORION–A at 22.233 GHz 1995 May 26 ORION at 22.235 GHz 1998 Oct. 14 10 4 0 0 –10 –4 10 0 –10 4 0 –4 ORION at 22.232 GHz 1999 Apr. 24 ORION at 22.235 GHz 1999 Aug. 11 4 4 0 0 –4 –4 4 0 –4 4 0 –4 Fig. 2. The structure of the H2O supermaser emission in the active region of Orion KL (scales in mas). The synthesized beam is shown in the lower left corner of each map; the epoch of observations is given above the maps. components with v =7.65 km s−1 by more than two THE ACTIVITY PERIOD 1998-1999 orders of magnitude. The intensity of the H2O maser emission ex- ponentially increased, reaching 4.3 MJy in Au- gust 1998, and began to exponentially decrease in THE EPOCH OF QUIESCENCE November. At the first stage of the outburst (March- October 1998), the structure remained virtually the On May 25, 1995, we investigated the region same as that in the quiescence period (Matveenko with an ultrahigh angular resolution of 0.1 mas, or et al. 1998), but the brightness of the components 0.04 AU (Matveenko et al. 1998, 2000, 2002). A × increased by more than three orders of magnitude, 9 0.05 AU jet and two compact “bullet” compo- ∼ 15 ∼ 17 nents were detected. A bright, compact source, the reaching Tjet = 10 KandTej = 10 K (Matveenko presumed ejector, and an elongated 0.5 × 0.04 AU et al. 2002). At the finalphase of activity in January – feature at a position angle of −44◦ with brightness April1999, the structure became more complex; ∼ ∼ additionalfeatures appeared in the centralregion, temperature T = 1013 KandT = 1012 Karelo- ej jet which can be part of a 1.2 × 0.8 AU torus ∼0.15 AU cated at the jet center (Fig. 2). The interaction of in thickness (Fig. 2). The radialout flow velocity bullets with the ambient medium, radiation pressure, ∼ −1 is v|| = 0.15 km s . The velocity in the plane of and stellar wind form a comet-like head–tailstruc- ∼ −1 ture. The tailcan be in front of or behind the head. the sky, v⊥ = 10 km s , is observed immediately ∼ The maser emission is linearly polarized, P = 55% before the outburst peak; at the peak, this velocity (Matveenko et al. 1998). decreased to 6 km s−1 and reached ∼3kms−1 at the ASTRONOMY LETTERS Vol. 29 No. 10 2003 THE STAR-FORMING REGION IN ORION KL 643 in diameter and 0.15 AU in thickness is located in the centralpart. The mass of the centralbody is (0.1–0.5)M. —The accretion and ejection of matter are differ- ent aspects of the same process. The ejector size is ∼ ≤0.05 AU, and the brightness temperature is Tb = 17 10 K. The ejected matter contains H2O molecules. The outflow velocity is ∼10 km s−1.Theoutflow collimation reaches ∼30. —The ejector rotation velocity is vrot ∼ 1.1 km s−1, and the rotation period is T ∼ 0.5 yr. The precession period reaches about 10 yr, and the precession angle is ∼30◦. The precession angle decreases by a factor of 2 at a distance of ∼1 AU, and the delay of rotation is 3.3 yr. Precession forms the helical structure of the outflow. —The bullets have comet-like head–tailstruc- tures. The emission of the structures is amplified in the surrounding envelope at velocity v =7.65 km s−1 −1 Fig. 3. A modelof the star-forming region. within a 0.5-km s -wide maser window. —The maser emission of the bipolar outflow is end of the activity period. The bipolar outflow has a pumped by collisions with the ambient medium while helical structure whose pitch and diameter increase the maser emission of the rings is associated with with distance from the ejector: T =1.3R0.7 mas, infrared radiation from the centralbody (IRc4). 0.8 ∅ =0.24R mas. The observed structuralchanges —The outburst polarization results from pumping are probably attributable to precession of the rotation anisotropy. The degree of polarization depends on the axis with a period of 110 months and a precession radiation flux density: P =(30+6.4 × F ) [%], where angle X ≈ 30◦. The intensity of the ejector emission F is in MJy. line seen at a velocity of 7.65 km s−1 in the ∆v = 0.42 km s−1 band is I =3× 105 Jy/beam. The line profile has broad wings, which are determined by the outflow rotation. The outflow radius is R =0.02 AU, REFERENCES and the rotation period is T ∼ 5 months. This profile 1. R. Bachiller, Ann. Rev. Astron. Astrophys. 34, 111 is determined by amplification in the envelope by a (1998). factor of approximately 450. 2. L. I. Matveenko, Pis’ma Astron. Zh. 7, 100 (1981) [Sov. Astron. Lett. 7, 54 (1981)]. 3.L.I.Matveenko,P.D.Diamond,andD.A.Graham, CONCLUSIONS Pis’ma Astron. Zh. 24, 723 (1998) [Astron. Lett. 24, Our studies of the active region in Orion KL with 623 (1998)]. an ultrahigh angular resolution reaching 0.1 mas or 4.L.I.Matveenko,P.D.Diamond,andD.A.Graham, 0.05 AU during 1979–1999 have shown the follow- Astron. Rev. 80, 592 (2000). ing: 5.L.I.Matveenko,P.D.Diamond,andD.A.Graham, —The structure at an early formation stage of a IAU Symp. 206: Cosmic Masers: from Protostars to low-mass star includes an accretion disk, a bipolar Blackholes, Ed.
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