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NEU17 - SOFIA/GREAT Discovery of Terahertz Water Masers

Short title: Neufeld et al. (2017)
Authors: Neufeld, David A.Melnick, Gary J.Kaufman, Michael J.Wiesemeyer, HelmutGÃ¼sten, RolfKraus, AlexMenten, Karl M.Ricken, OliverFaure, Alexandre
Journal: The Astrophysical Journal, Volume 843, Issue 2, article id. 94, 7 pp. (2017).
Bibcode:

Target type: star
Transitions: H2O_22,H2O_1278,H2O_1296,H2O_1884

Abstract
We report the discovery of water maser emission at frequencies above 1 THz. Using the GREAT instrument on SOFIA, we have detected emission in the 1.296411 THz {8}₂₇-{7}₃₄ transition of water toward three oxygen-rich evolved stars: W Hya, U Her, and VY CMa. An upper limit on the 1.296 THz line flux was obtained toward R Aql. Near-simultaneous observations of the 22.23508 GHz {6}₁₆-{5}₂₃ water maser transition were carried out toward all four sources using the Effelsberg 100 m telescope. The measured line fluxes imply 22 GHz/1.296 THz photon luminosity ratios of 0.012, 0.12, and 0.83, respectively, for W Hya, U Her, and VY CMa, values that confirm the 22 GHz maser transition to be unsaturated in W Hya and U Her. We also detected the 1.884888 THz {8}₄₅-{7}₅₂ transition toward W Hya and VY CMa, and the 1.278266 THz {7}₄₃-{6}₅₂ transition toward VY CMa. Like the 22 GHz maser transition, all three of the THz emission lines detected here originate from the ortho-H₂O spin isomer. Based upon a model for the circumstellar envelope of W Hya, we estimate that stimulated emission is responsible for ~85% of the observed 1.296 THz line emission, and thus that this transition may be properly described as a terahertz-frequency maser. In the case of the 1.885 THz transition, by contrast, our W Hya model indicates that the observed emission is dominated by spontaneous radiative decay, even though a population inversion exists.

Abstract

We report the discovery of water maser emission at frequencies above 1 THz. Using the GREAT instrument on SOFIA, we have detected emission in the 1.296411 THz {8}₂₇-{7}₃₄ transition of water toward three oxygen-rich evolved stars: W Hya, U Her, and VY CMa. An upper limit on the 1.296 THz line flux was obtained toward R Aql. Near-simultaneous observations of the 22.23508 GHz {6}₁₆-{5}₂₃ water maser transition were carried out toward all four sources using the Effelsberg 100 m telescope. The measured line fluxes imply 22 GHz/1.296 THz photon luminosity ratios of 0.012, 0.12, and 0.83, respectively, for W Hya, U Her, and VY CMa, values that confirm the 22 GHz maser transition to be unsaturated in W Hya and U Her. We also detected the 1.884888 THz {8}₄₅-{7}₅₂ transition toward W Hya and VY CMa, and the 1.278266 THz {7}₄₃-{6}₅₂ transition toward VY CMa. Like the 22 GHz maser transition, all three of the THz emission lines detected here originate from the ortho-H₂O spin isomer. Based upon a model for the circumstellar envelope of W Hya, we estimate that stimulated emission is responsible for ~85% of the observed 1.296 THz line emission, and thus that this transition may be properly described as a terahertz-frequency maser. In the case of the 1.885 THz transition, by contrast, our W Hya model indicates that the observed emission is dominated by spontaneous radiative decay, even though a population inversion exists.

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Detection table from Neufeld et al. (2017)

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ID	Obs. RA	Obs. Dec	l	b	Group id	Source name	Image	Line	V_peak	FWHM	∫F dv	Telescope	V_step	Beam	Detection	Class	Distance	Dist. (parallax)	Date
	(J2000)	(J2000)	(deg.)	(deg.)				H₂O	(km/s)	(km/s)	(Jy*km/s)		(km/s)	(arcsec)	Yes/No		(kpc)	(kpc)
1	07 22 58.3	-25 46 3.2	239.3526	-5.0654	G239.352-5.065	VY CMa	Image	1.27 THz;	19.7	10.95	10500 (790)	SOFIA	0.51	20	+H₂O -CH₃OH I -CH₃OH II +OH +SiO	EVOLVED	1.200		2017 Feb 01
2	13 49 02	-28 22 3.5	318.0223	32.8108	G318.022+32.811	W Hya	Image	1.29 THz;	38.7	3.26	6970 (330)	SOFIA	0.93	20	+H₂O +OH +SiO	EVOLVED	0.078		2016 May 27
3	16 25 47.5	+18 53 32.9	35.3451	40.3509	G35.345+40.351	U Her	Image	1.29 THz;	-15.4	1.73	1220 (170)	SOFIA	0.93	20	+H₂O +OH +SiO	EVOLVED	0.266		2016 May 26
4	07 22 58.3	-25 46 3.2	239.3526	-5.0654	G239.352-5.065	VY CMa	Image	1.29 THz;	19.6	8.77	11220 (520)	SOFIA	0.93	20	+H₂O -CH₃OH I -CH₃OH II +OH +SiO	EVOLVED	1.200		2017 Feb 01
5	13 49 02	-28 22 3.5	318.0223	32.8108	G318.022+32.811	W Hya	Image	1.88 THz;	39.7	3.66	3690 (530)	SOFIA	1.03	14	+H₂O +OH +SiO	EVOLVED	0.078		2016 May 27
6	16 25 47.5	+18 53 32.9	35.3451	40.3509	G35.345+40.351	U Her	Image	1.88 THz;	-16.3	1.68	550 (300)	SOFIA	1.03	14	+H₂O +OH +SiO	EVOLVED	0.266		2016 May 26
7	07 22 58.3	-25 46 3.2	239.3526	-5.0654	G239.352-5.065	VY CMa	Image	1.88 THz;	22.6	8.12	11080 (660)	SOFIA	1.03	14	+H₂O -CH₃OH I -CH₃OH II +OH +SiO	EVOLVED	1.200		2017 Feb 01
8	13 49 02	-28 22 3.5	318.0223	32.8108	G318.022+32.811	W Hya	Image	22 GHz;	38.7	1.85	86.5	EFF		41	+H₂O +OH +SiO	EVOLVED	0.078		2016 May 11
9	16 25 47.5	+18 53 32.9	35.3451	40.3509	G35.345+40.351	U Her	Image	22 GHz;	-14.6	1.17	167	EFF		41	+H₂O +OH +SiO	EVOLVED	0.266		2016 May 26
10	19 06 22.3	+08 13 46.9	41.9522	0.4537	G41.952+0.454	R Aql	Image	22 GHz;	46.5	1.08	46	EFF		41	+H₂O -CH₃OH I +OH +SiO	EVOLVED	0.214		2016 Nov 14
11	07 22 58.3	-25 46 3.2	239.3526	-5.0654	G239.352-5.065	VY CMa	Image	22 GHz;	15.1	7.93	9300	EFF		41	+H₂O -CH₃OH I -CH₃OH II +OH +SiO	EVOLVED	1.200		2017 Feb 01

Non-detection table from Neufeld et al. (2017)