Search for Compact Extragalactic Radio Sources Near Massive(2)

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

VLBIphase-referencedobservationsinvolveaphasecalibratorandatargetsource.How-ever,thedistributionofknowncalibratorsisnotsu cientlydenseenoughto ndonewithin≈2 ofanygivenmasersource,whichisnecessaryforhighprecisionastrometry:Systematicerrorsareusuallythelimitingfactorinphase-referencedpositionmeasurements,andtheseerrorsscalewiththeangularseparationbetweenthesources.Therefore, ndingcalibratorsascloseaspossibletothemasertargetsourcesismandatory.

Forthispurpose,wehaveconductedasurveyofNRAOVLASkySurvey(NVSS;Condonetal.1998)sourcesnearourmethanolmasertargets.Wechosesourceswhose1.4GHz uxdensitiesaregreaterthan20mJyandthatareunresolved(<20′′)attheNVSSresolution.TheNVSSsurveytypicallyyieldsabout15suchcompactsourceswithin1 ofanytargetposition.SinceourmasersourcesareintheGalacticplanemanyoftheseNVSSsourcesarecompactHIIregions,planetarynebulae(PNe),and,perhaps,compactsupernovaremnants(SNRs),and,thus,unsuitableforVLBIobservations.However,someoftheunresolvedsourcesarelikelyextragalacticsynchrotronsources.

InadditiontotheNVSSsources,weaugmentedourcandidatelistwithsourcesfromtheliterature(Gregory&Condon1991;Maetal.1998)andincludedknowncompactsourcesasacheckonourprocedures.Intotal,weobserved234compactsourceswithin1 ofthemasersources.

2.ObservationsandDatareductions

On2004April2wemadesnap-shotobservationsofour234sourcesusingtheNRAO1VLAinitsC-con gurationatC-band(4.885GHz)andU-band(14.939GHz).Bandwidthswere50MHz,observationswereindualcircularpolarization,andon-sourcetimeswerebetween30and70seconds.Theabsolute uxdensityscalewasdeterminedfromobservationsof3C286and3C48.Occasionallywemissedobservationsowingtoelectronicproblemsand,thus,forsomesourcesnodatacouldbeobtained(seeTable4).ThedatawerereducedwiththeNRAOAstronomicalImageProcessingSystem(AIPS)usingstandardprocedures.Weachievedrmsnoiselevelsofabout0.3mJyat4.885GHzand0.8mJyat14.939GHz,respectively.

Inordertoobtainbetterpositionalaccuracyandcheckoncompactness,were-observed53sourcesthathad uxdensitieshigherthan5mJyinboth,theC-andU-bandsandwere