Search for Compact Extragalactic Radio Sources Near Massive

We have used the Very Large Array to search for compact milliarcsecond-size radio sources near methanol masers in high-mass star-forming regions. Such sources are required for Very Long Baseline Interferometry phase-referencing observations. We conducted p

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002 nJu 61 1v8936060/hp-ortsa:viXraRevised:31May2006

Searchforcompactextragalacticradiosourcesnearmassivestar

formingregions

Y.Xu1,2,M.J.Reid3,K.M.Menten2,

andX.W.Zheng4ABSTRACT

WehaveusedtheVeryLargeArraytosearchforcompactmilliarcsecond-sizeradiosourcesnearmethanolmasersinhigh-massstar-formingregions.SuchsourcesarerequiredforVeryLongBaselineInterferometryphase-referencingob-servations.Weconductedpointedobservationsof234compactsourcesfoundintheNVSSsurveyand nd92sourceswithunresolvedcomponentsandsyn-chrotronspectralindexes.ThesesourcesarelikelythecoresofAGNsand,thus,goodcandidatesforastrometriccalibrators.

Subjectheadings:galaxies—radiocontinuum—astrometry

1.Introduction

Phase-referencedVeryLongBaselineInterferometry(VLBI)observationscanmeasureaccuratelythepositionofatargetsourcerelativetoareferencesource.This,forexample,permitsonetomeasurethetrigonometricparallaxandpropermotionsofsourcesinourgalaxyrelativetoextragalacticsources.WearenowcarryingoutalargeprogramtodothisformethanolmasersinregionsofhighmassstarformationthroughouttheMilkyWay.Table1contains38sourcesshowingmaseremissionin12.2GHz20 3 1Elineofmethanolthatweultimatelyintendtoobserve.