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Spatial and velocity coincidence of OH masers in two star-forming regions, Cep A and W75N.

Spatial and velocity coincidence of OH masers in two star-forming regions, Cep A and W75N. A. Niezurawska, M. Szymczak 1 , R.J. Cohen and A.M.S. Richards 2. 1 Torun Centre for Astronomy, Poland 2 Jodrell Bank Observatory, University of Manchester, UK. Cepheus A.

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Spatial and velocity coincidence of OH masers in two star-forming regions, Cep A and W75N.

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  1. Spatial and velocity coincidence of OH masers in two star-forming regions, Cep A and W75N. A. Niezurawska, M. Szymczak 1, R.J. Cohen and A.M.S. Richards 2 1 Torun Centre for Astronomy, Poland 2 Jodrell Bank Observatory, University of Manchester, UK

  2. Cepheus A • is a part of the Cepheus OB3 association (d=725 pc; Blaauw et al. 1959) • contains compact H IIregions (e.g. Garay et al. 1996; Torrelles et al. 1998) • and OH and H2O maser sources (e.g. Cohen et al. 1984; Argon et al. 2000) W75N • lies in the molecular cloud complex DR21-W75 (d=2 kpc; • Dickel et al. 1978) • contains compact H II regions (e.g. Hunter et al. 1994; Torrelles et al. 1997) • and OH and H2O maser sources (Baart et al. 1986; Hutawarakorn et al. 2002) Both sources showed 4765-MHz OH maser emission (Szymczak et al. 2000).

  3. Observations: • 5 or 6 antennas of MERLIN • bandwidth 250-kHz (128 channels) • phase – referencing technique • RMS noise = few mJy beam-1 • Cep A: • January – June 1999 • Freq.: 4660-, 4750-, 4765-MHz and 1612-, 1665-, 1667- • and 1720-MHz • W75N: • April 2000 • Freq. 4765-MHz

  4. 4765-MHz OH • Cep A • 4.9-GHz continuum • (Garay et al. 1996) • masers: • - 22-GHz H2O + • (Torrelles et al. 1998) • - 1665-MHz OH  • (Argon et al. 2000)

  5. 4765-MHz OH at Cep A: • three groups (A, B, C) with • well definded central VLSR • arc-like structure • the structure stable at all • epochs (8 weeks) • overall velocity gradient

  6. The I Stokes spectra of the 4765-MHz emission at three epochs: S4765 ~ t (-0.34±0.08)

  7. 4765-/1720-MHz coincidence at Cep A: 1. Spatial coincidence within 40 mas (group B). 2.Velocity coincidence:

  8. 4765-MHz OH • W75N • 8.4-GHz continuum • (Hunter et al. 1994) • masers: • 22-GHz H2O + • (Torrelles et al. 1997) • 1665-MHz OH  • (Hutawarakorn et al. 2002)

  9. 4765-MHz OH at W75N: • three groups (A, B, C) with • well definded central VLSR • N-S structure • overall velocity gradient

  10. The total flux density spectrum of the 4765-MHz OH from W75N: 1720-MHz OH spots (Hutawarakorn et al. 2002) coincide to within 60 mas and lie close (0.3 km s-1) to velocity of the feature A.

  11. Results: • spatial and velocity coincidence of 4765- and 1720-MHz OH • maser emission at Cep A and W75N, • both transitions arise in the same gas volume, • there is no other ground state OH lines above a sensitivity • level of 15 mJy beam-1. W75N Cep A

  12. Models: • S1720:S4765 = 25 (±5) at Cep A on 1999 May 20 (S4765 ~ t (-0.34±0.08)) • the model of Gray et al. (1991): nH2=4*106 cm-3 • nOH=200 cm-3 • TK=100-200 K • a large-scale velocity field • models of Pavlakis & Kylafis (1996) and Cragg et al. (2002) • poorly fit to our data • Dodson & Ellingsen (2002) reported a strong association • between 4765- and 6035-MHz OH. Baudry et al. (1997) • reported those masers at both sources, however there is no • available information where they are located.

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