Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.atnf.csiro.au/research/masermeeting/web_papers/304_Goedhart.pdf Дата изменения: Tue Apr 10 02:46:54 2007 Дата индексирования: Sun Dec 23 11:35:46 2007 Кодировка: Поисковые слова: gravitational radiation

Periodic variations in 6.7 GHz methanol masers

Sharmila Goedhart Mike Gaylard
Hartebeesthoek Radio Astronomy Observatory


6.7 GHz methanol masers can be highly variable
G351.78-0.54 ­ variations over one year showing three cycles. What can maser variability tell us about the process of high mass star formation?


Observations at 6.7 GHz
with the 26-m Hartebeesthoek Telescope



Initial survey

(Goedhart et al., 2004 MNRAS, 355,553) January 1999 --> April 2003 54 sources single polarisation (LCP) 256-channel correlator at 0.32 or 0.64 MHz bandwidth

:




Ongoing monitoring
New main reflecting surface, ~0.5mm rms (22 GHz available!) Rebuilt receiver in 2003, dual polarisation, LCP & RCP new control computer & observing software 1024 channels/pol correlator at 1 MHz bandwidth 19 sources of interest


Amplitude calibration



Continuum drift scans scheduled with each set of spectral line observations Hydra A, Virgo A & 3C123 with pointing checks





Some periodic maser candidates


G338.93-0.06: Range of variation


G338.93-0.06: Time-series


G338.93-0.06: Epoch-folding & Fourier Periodograms

P = 133 days


G338.93-0.06: Time-series folded modulo 133 days


G331.13-0.24: Averaged spectrum


G331.13-0.24: Time-series


G331.13-0.24: Epoch-folding & Fourier Periodograms

P = 504 days


G331.13-0.24: Sinusoid fits, P=504d


G328.24-0.55: Range of variation


G328.24-0.55: Time-series


G328.24-0.55: Epoch-folding & Fourier Periodograms

P = 220 days


G339.62-0.12: Range of variation


G339.62-0.12: Time-series


G339.62-0.12: Epoch-folding & Fourier periodograms

P = 201 days


G9.62+0.20: Range of variation


G9.62+0.20: Time-series


G9.62+0.20: Epoch-folding & Fourier periodograms

P = 244 days


G188.95+0.89: Range of variation


G188.95+0.89: Time-series


G188.95+0.89: Sinusoidal fits with P=404 days


So, what happens during a flare?



Local effect or increase in incoming radiation? Do new spots appear? Is it a shock wave or some other disturbance passing through the masing regions?




VLBA observations of G9.62+0.20


7 observations during October ­ December 2001 6 hours integration time/observation 12.2 GHz Goedhart, Minier, Gaylard & van der Walt, 2005, MNRAS, 356, 839






G9.62+0.20 @ 12 GHz during a flare

Available on-line at MNRAS


Causes of periodicity


Rotating hot spots? Rotation periods ~ days Stellar pulsations? Beta Cep periods < 1 day Binary system?
- - - -

Periods imply orbital radii between ~ 1 ­ 10 AU Solid angles of eclipsing binaries probably too small Maybe one or both stars still have an accretion disk? Accretion from companion ­ cataclysmic variables


Causes of periodicity, cont.


Precessing jets? Accretion disks?
- - -

gravitational instabilities condensations periodic mass dumps (binary)



???


Summary


8 year time-series Six (five?) periodic maser candidates Period range: 133d ­ 504d Varying amplitudes & durations in flares but start times the same Behaviour may evolve over time




Outlook


Lots of work to do! We need a time-dependent maser model Can we distinguish between possibilities of a binary system, precessing jet or accretion disk? Radiative transfer calculations ­ cause of disturbance at a few AU but masers at 100s ­ 1000s AU