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SUG08 - A Synchronized Variation of the 6.7GHz Methanol Maser in Cepheus A

Short title: Sugiyama et al. (2008)
Authors: Sugiyama, KoichiroFujisawa, KentaDoi, AkihiroHonma, MarekiIsono, YasukoKobayashi, HideyukiMochizuki, NanakoMurata, Yasuhiro
Journal: Publications of the Astronomical Society of Japan, Vol.60, No.5, pp.1001--1006
Bibcode:

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Detection table: Show detection table (1 detection)
Target type: Known_Class_II
Transitions: CH3OH_6(II)

Abstract
We present the results of monitoring observations of a 6.7GHz methanol maser in Cepheus A (Cep A) using the Yamaguchi 32m radio telescope and of imaging observations conducted with the JVN (Japanese VLBI Network). We identified five spectral features which are grouped into two groupes: redshifted (-1.9 and -2.7 km s^-1) and blueshifted (-3.8, -4.2, and -4.9 km s^-1). We detected rapid variabilities in these maser features within a monitoring period of 81d. The redshifted features decreased in flux density to 50% of the initial value, while the blueshifted ones rapidly increased within 30d. The time variation in these maser features had two remarkable properties: synchronization and negative correlation between the redshifted and the blueshifted. Based on the JVN observations, we found that the maser spots were associated with the CepAHW2 object and had an arched structure with a scale of 1400AU also, separations of the five maser features were found to be larger than 100AU. These properties of the masers, namely, the synchronization of the flux variation and the spectral and spatial isolations of the features, suggest that collisional excitation by a shock wave from a common exciting source is unlikely to happen. Instead, the synchronized time variation of the masers can be explained if all of the maser features are excited by infrared radiation from the dust that is heated by a common exciting source with a rapid variability.

Abstract

We present the results of monitoring observations of a 6.7GHz methanol maser in Cepheus A (Cep A) using the Yamaguchi 32m radio telescope and of imaging observations conducted with the JVN (Japanese VLBI Network). We identified five spectral features which are grouped into two groupes: redshifted (-1.9 and -2.7 km s^-1) and blueshifted (-3.8, -4.2, and -4.9 km s^-1). We detected rapid variabilities in these maser features within a monitoring period of 81d. The redshifted features decreased in flux density to 50% of the initial value, while the blueshifted ones rapidly increased within 30d. The time variation in these maser features had two remarkable properties: synchronization and negative correlation between the redshifted and the blueshifted. Based on the JVN observations, we found that the maser spots were associated with the CepAHW2 object and had an arched structure with a scale of 1400AU also, separations of the five maser features were found to be larger than 100AU. These properties of the masers, namely, the synchronization of the flux variation and the spectral and spatial isolations of the features, suggest that collisional excitation by a shock wave from a common exciting source is unlikely to happen. Instead, the synchronized time variation of the masers can be explained if all of the maser features are excited by infrared radiation from the dust that is heated by a common exciting source with a rapid variability.

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Detection table from Sugiyama et al. (2008)

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ID	Obs. RA	Obs. Dec	l	b	Group id	Source name	Image	Descr.	Line	V_peak	Telescope	Beam	Detection	Class	Dist. (parallax)	Date
	(J2000)	(J2000)	(deg.)	(deg.)					CH₃OH (class II)	(km/s)		(arcsec)	Yes/No		(kpc)
1	22 56 17.9	+62 01 49.6	109.8709	2.1144	G109.871+2.114	Cep A	Image	Descr	6.6 GHz (II);	-2.64	YAM	300	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		2007 August - October

Distribution of sources in all sky (l,b)

Distribution of sources in all sky using Aladin

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Distribution of sources in IRAS color-color diagram

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Original color diagram is taken form van der Veen & Habing (1987)

Type	Description
I	Oxygen-rich non-variable stars without circumstellar shells
II	Variable stars with "young" O-rich circumstellar shells
IIIa	Variable stars with more evolved O-rich circumstellar shells
IIIb	Variable stars with thick O-rich circumstellar shells
IV	Variable stars with very thick O-rich circumstellar shells
V	Planetary nebulae and non- variable stars with very cool circumstellar shells
VIa	Variable stars with relatively cold dust at large distances; a significant part of the stars is carbon-rich
VIb	Variable stars with relatively hot dust close to the star and relatively cold dust at large distances; Some of the objects have proven to be O-rich
VII	Variable stars with more evolved C-rich circumstellar shells
VIII	Different sort of objects

Distribution of the detection velocities versus galactic longitude from SUG08

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