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XU18 - The Parallax of the Red Hypergiant VX Sgr with Accurate Tropospheric Delay Calibration

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Short title: Xu et al. (2018)
Authors: Xu, ShuangjingZhang, BoReid, Mark J.Menten, Karl M.Zheng, XingwuWang, Guangli
Journal: The Astrophysical Journal, Volume 859, Issue 1, article id. 14, 10 pp. (2018).
Bibcode:

Target type: star

Abstract
We report astrometric results of VLBI phase-referencing observations of 22 GHz H2O masers emission toward the red hypergiant VX Sgr, one of most massive and luminous red hypergiant stars in our Galaxy, using the Very Long Baseline Array. A background source, J1820-2528, projected 4.�4 from the target VX Sgr, was used as the phase reference. For the low decl. of these sources, such a large separation normally would seriously degrade the relative astrometry. We use a two-step method of tropospheric delay calibration, which combines the VLBI geodetic-block (or Global Positioning System) calibration with an image-optimization calibration, to obtain a trigonometric parallax of 0.64 � 0.04 mas, corresponding to a distance of {1.56}-0.10+0.11 kpc. The measured proper motion of VX Sgr is 0.36 � 0.76 and -2.92 � 0.78 mas yr-1 in the eastward and northward directions. The parallax and proper motion confirms that VX Sgr belong to the Sgr OB1 association. Rescaling bolometric luminosities in the literature to our parallax distance, we find that the luminosity of VX Sgr is (1.95 � 0.62) × 105 L ⊙, where the uncertainty is dominated by differing photometry measurements.

Abstract

We report astrometric results of VLBI phase-referencing observations of 22 GHz H2O masers emission toward the red hypergiant VX Sgr, one of most massive and luminous red hypergiant stars in our Galaxy, using the Very Long Baseline Array. A background source, J1820-2528, projected 4.�4 from the target VX Sgr, was used as the phase reference. For the low decl. of these sources, such a large separation normally would seriously degrade the relative astrometry. We use a two-step method of tropospheric delay calibration, which combines the VLBI geodetic-block (or Global Positioning System) calibration with an image-optimization calibration, to obtain a trigonometric parallax of 0.64 � 0.04 mas, corresponding to a distance of {1.56}-0.10+0.11 kpc. The measured proper motion of VX Sgr is 0.36 � 0.76 and -2.92 � 0.78 mas yr-1 in the eastward and northward directions. The parallax and proper motion confirms that VX Sgr belong to the Sgr OB1 association. Rescaling bolometric luminosities in the literature to our parallax distance, we find that the luminosity of VX Sgr is (1.95 � 0.62) × 105 L ⊙, where the uncertainty is dominated by differing photometry measurements.

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Detection table from Xu et al. (2018)

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ID	Obs. RA	Obs. Dec	RA offset	Dec offset	l	b	Group id	#	Maser spot	Source name	Image	Descr.	Line	V_peak	μ_X	μ_Y	π	F_peak	Telescope	Beam	PA	Detection	Class	Dist. (parallax)	Date
	(J2000)	(J2000)	(mas)	(mas)	(deg.)	(deg.)							H₂O	(km/s)	(mas/yr)	(mas/yr)	(mas)	(Jy/beam)		(arcsec)	(deg.)	Yes/No		(kpc)
1	+18 08 4.051	-22 13 26.566			8.3441329	-1.0019495	G8.344-1.002			VX Sgr	Image	Descr	22 GHz;	1.2				60	VLBA	0.7 x 0.3	-5	+H₂O +OH +SiO	SFR_MIXED	1.56 (-0.10 +0.11)	2009 Sep - 2010 Sep
	+18 08 4.04826333	-22 13 26.56212	-41.17 mas	3.84 mas	8.344129	-1.001940		1	A	VX Sgr				22.6	0.58 (0.14)	-0.32 (0.32)	0.623 (0.056)
	+18 08 4.04826733	-22 13 26.56278	-41.11 mas	3.18 mas	8.344129	-1.001940		2	A	VX Sgr				21.8	0.27 (0.07)	-1.02 (0.23)	0.668 (0.025)
	+18 08 4.049674	-22 13 26.55617	-20.01 mas	9.79 mas	8.344133	-1.001944		3	B	VX Sgr				9.6	0.68 (0.16)	-0.95 (0.25)	0.615 (0.065)
	+18 08 4.049634	-22 13 26.55678	-20.61 mas	9.18 mas	8.344133	-1.001944		4	B	VX Sgr				9.2	0.59 (0.10)	-1.07 (0.54)	0.657 (0.039)
	+18 08 4.03980733	-22 13 26.58925	-168.01 mas	-23.29 mas	8.344106	-1.001915		5	C	VX Sgr				5.8	-1.65 (0.22)	-1.89 (0.21)	0.694 (0.091)
	+18 08 4.041958	-22 13 26.63806	-135.75 mas	-72.1 mas	8.344098	-1.001929		6	D	VX Sgr				4.9	-1.41 (0.12)	-2.98 (0.32)	0.624 (0.046)
	+18 08 4.04195	-22 13 26.6373	-135.87 mas	-71.34 mas	8.344099	-1.001929		7	D	VX Sgr				4.5	-1.52 (0.10)	-3.08 (0.38)	0.653 (0.041)
	+18 08 4.041886	-22 13 26.63704	-136.83 mas	-71.08 mas	8.344099	-1.001928		8	D	VX Sgr				4.1	-1.28 (0.05)	-2.81 (0.34)	0.562 (0.017)
	+18 08 4.050322	-22 13 26.56792	-10.29 mas	-1.96 mas	8.344131	-1.001947		9	E	VX Sgr				1.6	0.56 (0.09)	-1.11 (0.34)	0.579 (0.037)
	+18 08 4.050964	-22 13 26.56578	-0.66 mas	0.18 mas	8.344133	-1.001949		10	F	VX Sgr				1.2	0.65 (0.08)	-0.97 (0.28)	0.650 (0.031)
	+18 08 4.05097133	-22 13 26.56565	-0.55 mas	0.31 mas	8.344133	-1.001949		11	F	VX Sgr				0.7	0.69 (0.10)	-0.90 (0.25)	0.644 (0.040)
	+18 08 4.04886733	-22 13 26.55129	-32.11 mas	14.67 mas	8.344132	-1.001940		12	G	VX Sgr				0.3	-0.01 (0.09)	-0.82 (0.38)	0.655 (0.036)
	+18 08 4.048912	-22 13 26.54961	-31.44 mas	16.35 mas	8.344133	-1.001940		13	G	VX Sgr				-0.4	0.06 (0.09)	-0.78 (0.40)	0.633 (0.037)
	+18 08 4.043076	-22 13 26.6764	-118.98 mas	-110.44 mas	8.344091	-1.001938		14	H	VX Sgr				-7.2	-1.55 (0.07)	-3.51 (0.35)	0.650 (0.027)
	+18 08 4.04307333	-22 13 26.67665	-119.02 mas	-110.69 mas	8.344091	-1.001938		15	H	VX Sgr				-7.6	-1.48 (0.04)	-3.50 (0.37)	0.656 (0.014)
	+18 08 4.04307133	-22 13 26.67683	-119.05 mas	-110.87 mas	8.344091	-1.001938		16	H	VX Sgr				-8.0	-1.50 (0.04)	-3.56 (0.36)	0.648 (0.015)

DSS colored	2MASS colored	WISE colored
SDSS color	IRAC color I1,I2,I4	GLIMPSE 360
PACS color (70-160 um)	SPIRE color (250-500 um)	NVSS (1.4GHz)
ATLASGAL 870 um	ATLASGAL 870 um + Planck	Bolocam 1.1 mm
UKIDSS K Download UKIDSS K FITS	2MASS K Download 2MASS K FITS

select MAX(h2o_data_all.ra_hms) as ra_hms,MAX(h2o_data_all.dec_dms) as dec_dms,MAX(h2o_data_all.source_name) as source_name,MAX(h2o_data_all.ra) as ra,MAX(h2o_data_all.dec) as dec,MAX(h2o_data_all.l) as l,MAX(h2o_data_all.b) as b,MAX(h2o_data_all.grp) as grp,MAX(h2o_data_all.other_name) as other_name,MAX(h2o_data_all.other_desc) as other_desc,BOOL_OR(thermal) as thermal,MAX(h2o_data_all.line) as line,MAX(h2o_data_all.vrange) as vrange,MAX(h2o_data_all.vpeak) as vpeak,MAX(h2o_data_all.peak) as peak,MAX(h2o_data_all.fwhm) as fwhm,MAX(h2o_data_all.int) as int,MAX(h2o_data_all.rms) as rms,MAX(h2o_data_all.units) as units,MAX(h2o_data_all.units_rms) as units_rms,MAX(h2o_data_all.date_txt) as date_txt,MAX(h2o_data_all.telescope_ref) as telescope_ref,MAX(h2o_data_all.telescope_conf) as telescope_conf,MAX(h2o_data_all.v_step) as v_step,MAX(h2o_data_all.beam) as beam,MAX(h2o_data_all.beam_maj) as beam_maj,MAX(h2o_data_all.beam_min) as beam_min,MAX(h2o_data_all.beam_pa) as beam_pa,MAX(h2o_data_all.ref) as ref,MAX(h2o_data_all.list_ref) as list_ref,MAX(h2o_data_all.ascii) as ascii,MAX(h2o_data_all.url) as url,MAX(h2o_data_all.image) as image,MAX(h2o_data_all.image2) as image2,MAX(h2o_data_all.image3) as image3,MAX(h2o_data_all.descr) as descr,MAX(h2o_data_all.id) as id,BOOL_OR(mjy_flag) as mjy_flag,BOOL_OR(rms_mjy_flag) as rms_mjy_flag,MAX(h2o_data_all.dist) as dist,1 as dist_parallax,MAX(h2o_data_all.date_txt) as date_txt,MAX(line_det) as line_det,MAX(type) as type,MAX(simbad_types_cat) as simbad_types_cat FROM h2o_data_all  LEFT JOIN all_objects_withmet ON h2o_data_all.grp=all_objects_withmet.group_name LEFT JOIN object_categories ON all_objects_withmet.group_name = object_categories.group_name_cat AND object_categories.tbl_cat = 'all_objects_withmet' LEFT JOIN transitions ON h2o_data_all.line=transitions."Line"  WHERE ref ilike 'XU18' AND detected=true AND hide=false GROUP BY ref

select h2o_data_all.ra_hms,h2o_data_all.dec_dms,h2o_data_all.source_name,h2o_data_all.ra,h2o_data_all.dec,h2o_data_all.l,h2o_data_all.b,h2o_data_all.grp,h2o_data_all.other_name,h2o_data_all.other_desc,h2o_data_all.thermal,h2o_data_all.line,h2o_data_all.vrange,h2o_data_all.vpeak,h2o_data_all.peak,h2o_data_all.fwhm,h2o_data_all.int,h2o_data_all.rms,h2o_data_all.units,h2o_data_all.units_rms,h2o_data_all.date_txt,h2o_data_all.telescope_ref,h2o_data_all.telescope_conf,h2o_data_all.v_step,h2o_data_all.beam,h2o_data_all.beam_maj,h2o_data_all.beam_min,h2o_data_all.beam_pa,h2o_data_all.ref,h2o_data_all.list_ref,h2o_data_all.ascii,h2o_data_all.url,h2o_data_all.image,h2o_data_all.image2,h2o_data_all.image3,h2o_data_all.descr,h2o_data_all.id,h2o_data_all.mjy_flag,h2o_data_all.rms_mjy_flag,h2o_data_all.dist,h2o_data_all.dist_parallax,h2o_data_all.date_txt,line_det,type,simbad_types_cat FROM h2o_data_all   LEFT JOIN all_objects_withmet ON h2o_data_all.grp=all_objects_withmet.group_name LEFT JOIN object_categories ON all_objects_withmet.group_name = object_categories.group_name_cat AND object_categories.tbl_cat = 'all_objects_withmet' LEFT JOIN transitions ON h2o_data_all.line=transitions."Line"  WHERE ref ilike 'XU18' AND detected=true AND hide=false ORDER BY detected DESC,id

Display data from paper

XU18 - The Parallax of the Red Hypergiant VX Sgr with Accurate Tropospheric Delay Calibration

Interferometry data

View as list of objects

Detection table from Xu et al. (2018)