A search for 53 MHz OH line near G48.4$-$1.4 using the National MST Radar Facility

0.18 MB·English

by  Srikumar M. Menon

Mon.Not.R.Astron.Soc.000,1–6() Printed2February2008 (MNLATEXstylefilev2.2)
A search for 53 MHz OH line near G48.4−1.4 using the
National MST Radar Facility
Srikumar M. Menon1⋆, D. Anish Roshi2† and T. Rajendra Prasad3‡
1Manipal Institute of Technology, Manipal 576 104, India
5 2Raman Research Institute, Sadashivanagar, Bangalore 560 080, India
0 3National MSTRadar Facility, Gadanki 517 112, India
0
2

n
a Accepted 2004October 12.Received2004October 6;inoriginalform2004August3
J

9
2 ABSTRACT
We presentthe results ofa searchfor the groundstate hyperfine transitionof the OH

1 radical near 53 MHz using the National MST Radar Facility at Gadanki, India. The
v observedpositionwasG48.4−1.4neartheGalacticplane.TheOHlineisnotdetected.
9 We place a 3σ upper limit for the line flux density at 39 Jy from our observations.
4
We also did not detect recombination lines (RLs) of carbon, which were within the
6
frequency range of our observations. The 3σ upper limit of 20 Jy obtained for the
1
flux density of carbon RL, along with observations at 34.5 and 327 MHz are used
0
5 to constrain the physical properties of the line forming region. Our upper limit is
0 consistentwiththelineemissionexpectedfromapartiallyionizedregionwithelectron
/ temperature, density and path lengths in the range 20 – 300 K, 0.03 – 0.3 cm−3 and
h
0.1 – 170 pc respectively (Kantharia & Anantharamaiah 2001).
p
-
Key words: ISM:general – ISM:lines and bands – ISM:molecules – radio lines:ISM
o
r – radio lines:general – Galaxy:general.
t
s
a
:
v
i 1 INTRODUCTION interstellar medium where OH lines originate. Despite the
X
large amount of observational data available and consider-
r Alargevarietyofmolecules,includingcomplexorganicones,
a abletheoreticaleffort,aclearunderstandingofthepumping
havebeendetectedinthegalactic interstellar medium.The
mechanisms that lead to different inversions in OH masers
hydroxyl (OH) radical is quite abundant in the galactic
is lacking (Elitzur 1976, Cesaroni & Walmsley 1990).
plane and has several rotational transitions that are eas-
ilyobservedatmicrowavefrequencies.Theselinesarefound Inaddition tothemicrowavelines, theOHradical also
to originate from thermal processes as well as non-thermal hastransitions inthemeter-wave.Theselines areproduced
due to transitions between hyperfine levels in the same λ
processes (i.e. maser emission). Thermal emission from OH
radical was first detected in 1963 (Weinreb et al. 1963). doubletstates(seeSection2).Thefrequenciesofthesetran-
sitions in the ground rotational state are 53 and 55 MHz.
Thethermallinesareobservedfromextendedregionsinthe
These lines, which have not been observed so far, are ex-
Galacticplane.Ontheotherhand,maseremissionfromOH
pected to have weak line intensities because they are mag-
radical is associated with specific activities in the Galaxy.
netic dipole transitions. Moreover, observations are further
For instance, OH masers from the ground state rotational
complicated duetoexcessiveman-maderadio frequencyin-
transitions with frequencies1665 and 1667 MHzare mostly
terference (RFI) near the line frequencies. It is owing to
associated with star-forming regions, 1612 MHz masers are
severe RFI at this frequency, along with the weak nature
associated with evolved stars (Elitzur 1992; Reid & Moran
of these lines, that attempts to detect these lines were not
1981)andthe1720MHzmasersareassociatedwithshocked
made earlier (Turner, B. E. personal communication). As
regions at the boundaries where supernova remnants inter-
discussed above, in a variety of astrophysical situations,
act with molecular clouds (Wardle & Yusuf-Zadeh 2002).
maser emission is observed from the microwave transitions.
Modelingthelineemissionprovidesanunderstandingofthe
Therefore, the possibility of maser emission of meter-wave
physicalconditions and processes that occur in thegalactic
transitions cannot be ruled out and may be strong enough
to be detected. The detection of these lines could provide
clues to resolve, for example, the pumping mechanism of
⋆ E-mail:[email protected]
OH masers observed at microwave frequencies.
† E-mail:[email protected]
‡ E-mail:trp1910@rediffmail.com In this paper, we present an attempt to detect the 53
2 S. M. Menon et. al.
53 MHz transition of ∼ 10−18 s−1, the rotational constant
of 18.51 cm−1 and considering a typical OH line width of 5
km s−1(Weaver 1963, Destombes et al. 1977, Turner1979).
A mean galactic background temperature of 25000 K at 53
F MHz is used for the calculation. This background temper-
ature is obtained by scaling the measured temperature at
2
+ 34.5 MHz using a spectral index of −2.7 (Dwarakanath &
55 Udaya Shankar 1990, Salter & Brown 1988) The expected
1 linebrightnesstemperatureis10−3 and1Kforcolumnden-
1720 1667 sities1014and1017cm−2respectively.Duetothehighgalac-
J = 3/2 1665 1612 tic background near 53 MHz (which dominates the system
temperature) it is not possible to detect the thermal OH
line in a reasonable observing time. However,there exists a
2
possibility of maser emission of the meter-wave transitions
53
− (Turner,B.E.personalcommunication).Toourknowledge,
1
there have been no attempts to calculate the line strengths
ofthesemaserlines.Wedecidedtosearchfor53MHzmaser
emission towardsdirectionwhereOHmicrowavemasers(as
well as thermal lines) were detected earlier.
Figure1.Energydiagramofthehyperfinelevelsfortheground
staterotationaltransitionsoftheOHradical.Thetransitionfre-
3 OBSERVATIONS
quencies aregiveninMHz.
Theobservationspresentedinthispaperaremadewiththe
NationalMSTRadarFacilitylocatedatGadankiinAndhra
MHz OH line by observing with the National MST Radar Pradesh, India (13o27′ 21.”12N, 79o10′ 37.”10E). In this
Facility(NMRF)atGadanki,India,inthereceivingmodeof section, we briefly describe the radar facility, the back-end
theantenna.Abriefdiscussionofthe53MHzOHlineemis- used for the observations and the selection of sources for
sion is given in Section 2. We describe the observing setup observation.
and strategy in Section 3 and discuss the data analysis in
Section4.ResultsoftheOHlineobservationsarepresented
in Section 5.Inaddition totheOHline,carbon recombina- 3.1 The MST Radar Facility
tionlines(RLs)werealsopresentwithintheobservingband.
AsimplifiedblockdiagramofthereceiversystemoftheMST
Theresultsof theRLobservationsarealso includedinSec-
RadarFacilityisshowninFig.2(seeRaoetal.1995forde-
tion5.Ourconclusionandprospectsforfutureobservations
tails). The antenna used for transmission and reception of
are given in Section 6.
radar signals is a phased array with 1024 crossed Yagi ele-
ments, placed in a 32 x 32 matrix. The array is spread out
over an area of 130m x 130m and has an effective aperture
of 104 m2. The antenna and receiver system are tuned to
2 53 MHZ OH LINE EMISSION
operate at a center frequency of 53 MHz with a bandwidth
The energy diagram for the ground state rotational transi- of ∼ 2 MHz. The two crossed Yagi elements, correspond-
tions of theOH molecule is shown in Fig. 1. The rotational ing to the two orthogonal linear polarizations, are oriented
ladder of OH is split into 2Π1 and 2Π3 ladders because of along the East-West (E-W) and North-South (N-S) direc-
2 2
thespin-orbit coupling ofasingle unpaired electron in a2p tions. The signals from the elements are combined to form
orbital on the O atom. Here we consider the ground rota- beams that can be pointed in steps of 1o to a maximum of
tional state, characterized byJ = 3.This state is split into 20o from the zenith along E-W and N-S directions. The E-
2
two levels as a result of the small difference in moment of Wbeamsareformed inthefollowing way.Thesignalsof32
inertiawhentheelectronorbitalisparallelorperpendicular N-S Yagi elements, which form a linear sub-array,are com-
tothemolecule’srotationaxis(λdoubling)andfurthersplit bined using passive directional couplers. Such a sub-array
by the hyperfine interaction between the electron spin and is also called a module. The output of 32 such modules are
thespinoftheprotonintheHatom.Themicrowavelinesof amplified using low-noise amplifiers and down converted to
OHatfrequencies1612,1665,1667and1720MHzarisefrom 5 MHz intermediate frequency (IF) using a local oscillator
transitionsbetweenthesefourλdoublingstates,i.e.,J = 3, (LO) of frequency 48 MHz. These IFoutputsare combined
2
F+ →F−, where + and − indicate the higher and lower λ with appropriate phase gradients to form the E-W beams.
doubletstates.(seeFig.1).Thetwomagneticdipoletransi- The phase-shifters are placed in the LO path as shown in
tionsfromquantumstatesF =2+ →1+ andF =2− →1− Fig. 2. Signals from the Yagi elements oriented in the E-W
havefrequencies near 55 and 53 MHz respectively. direction are combined in asimilar fashion to form theN-S
We estimate the thermal line intensity from a cloud beams. The half power beam width of the antenna is ∼3o.
with OH column density in the range 1014 to 1017 cm−2 TheE-Wbeamscanbepointedinstepsof1otoamaximum
and a typical kinetic temperature of 100 K (Weinreb et al. of20ofromthezenith.Forourobservations,weusedthedif-
1963, Elitzur 1976, Lockett et al. 1999). The line optical ferentE-Wbeamstotrackthecelestialsource.Notethatthe
depth is estimated using the Einstein A coefficient for the MSTarrayisorientedtothegeomagneticmeridianwhichis
53 MHz OH line 3
LO1(48MHz)
Phase Shifter
Signal from the antenna (5MHz)
IF Bit
LO2 Sampler Packing
1 Module
Clock Device
Yag3i2 A xn 3te2nna 53MHz 5MHz Power To backend Synthesizer 15.7MHz DAS
Combiners Rubidium (PC)
Array
32 Oscillator
Clock
Synthesizer LO1 to MST (47.8948MHz)
Phase Shifter
LO1(48MHz)
Figure 2. Blockdiagram of the receiver system of the National Figure 3.Blockdiagramoftheback-endusedforobservations.
MSTRadarFacility.
theRL.Themodifieddataacquisitionsoftwaresetsthefirst
oriented2odueWestwithreferencetothegeographicmerid- local oscillator frequency and acquires 640 KBytes of data
ianatGadanki.Thisorientationofthearrayalongwiththe corresponding to each frequency setting after allowing for
location of the facility at a latitude of ∼13o.5 makes the settling time of a few msec. This amount of data will be
source move away from the beam centre while tracking the acquiredinaboutasec.Thusthefrequencyswitchingtakes
source using the E-W beams (see Section. 3.4 for further place at 1 sec rate.
discussion).
3.4 Observing Strategy and Source Selection
3.2 The back-end
The candidate sources towards which the 53 MHz OH line
ThePortablePulsarReceiver(PPR;Deshpandeetal.2004) emission is expected are mostly in the Galactic plane. The
developedattheRamanResearchInstitutewasusedasthe long integration required for our observations can be ob-
backendfortheobservations.TheblockdiagramofthePPR tained if the antenna could track the celestial source. How-
isshown inFig. 3.The5MHzIFfrom theradarreceiveris ever,theMSTradarfacilityinitscurrentconfigurationdoes
mixedwithalocaloscillatoroffrequency15.7MHztoobtain notpermittrackingofcelestialsources.Weadoptedastrat-
a second IF at 10.7 MHz. The local oscillator is generated egy of observing a position in the Galactic plane with the
using a synthesizer in the PPR and its frequency can be different East-West beams of the antenna, thus effectively
programmed from the computer used for data acquisition. trackingthepositionforabout40o(seeSection3.1).Thispo-
The bandwidth of the second IF is restricted to ∼1.3 MHz sitioncorrespondstothegalacticco-ordinatesl=48o.4and
usingabandpassfiltercentredat10.7MHz.TheIFsignalis b = −1o.4 (RA(2000) = 19h25m33.0s, Dec(2000) = +13o
bandpasssampledat3.3MHzandconvertedto2-bit,4-level 06′ 35.2′′). Note that G48.4−1.4 is the only position in the
data. A bit-packing logic then packs consecutive samples innerGalaxy that can be tracked for 40oin thismode using
into 16-bit words. This data is acquired and stored using thepresent configuration of theMST radar.
a personal computer. A marker is also inserted after 4096
MicrowavemasersofOHhavebeendetectedwithinthe
data words to identify any loss of samples while analysing effective beam area (∼ 4o; see below) of the observation
data. The PPR has another synthesizer which we used to centered at G48.4−1.4 (see Table 1). Most of these masers
generate the first local oscillator signal (see below). Both
are associated with the star forming region W51 (Gaume
thesynthesizers are locked to a rubidium oscillator. & Mutel 1987). All the four OH lines were detected from
thisregion andtypicallythemain lines(1665 &1667MHz)
areseentobebrightinemission.Anomalous1720MHzOH
3.3 Modification of PPR data acquisition software
maseremission associated withshockedmoleculargasadja-
TheexistingdataacquisitionsoftwareforthePPRwasmod- cent to the supernova remnant G49.2−0.7 was also present
ifiedtoincorporateadual-Dickefrequencyswitchingscheme within the beam area (Frail et al.1996). Moreover, Roshi &
forbandpasscalibration. Weswitched thefirstlocal oscilla- Anantharamaiah (2000) have detected carbon RLs towards
tor(LO1) between 47.8948 MHzand48.0 MHz.Thevalues G50+0.0withabeamof2o.Thuswedecidedtoobservethe
of the frequencies were selected so that the 53 MHz line of direction G48.4−1.4 with theMST radar facility.
OH and five RL of carbon be within the ∼1.3 MHz band- The direction G48.4−1.4 was observed in a total of 13
widthusedforobservations.Thedifferencebetweenthetwo East-West beams separated by the half-power beam-width
frequencies was chosen based on the expected line width of of 3o, resulting in an integration time of approximately 2.5
4 S. M. Menon et. al.
Table 1.OHmaserandRLsourceswithin4o ofG48.4−1.4. hours)waslessthanthespectralresolution.Inthefrequency
switching scheme (double-Dicke frequency switching) used
Source Type Reference forobservations,theexpectedlinesarepresentattwospec-
tral ranges in the calibrated spectrum separated by 105.2
W51 OHMaser Gaume&Mutel(1987) kHz.Theaveragespectrumisobtainedbyfoldingthesetwo
G49.2−0.7 Anomalous1720MHz Frailetal.(1996)
frequencyranges.Thefoursuchspectraobtainedfromeach
maser
day’s observing run were then averaged after applying the
G50+0.0 CarbonRL Roshi&
required correction for the LSR motion of the antenna to
Anantharamaiah(2000)
get thefinal spectrum.
Duringdataanalysis,wefoundthatmostoftheinterfer-
hours on each observing run. As discussed in Section 3.1, encewasinternaltotheMSTradar.Theseinterferencewere
inthismodeofobservation,theskyposition shiftsfromthe causedbyoscillationsfromsomeofthefront-endamplifiers.
beamcenterbyabout±0o.4inRAand±0o.75inDec.Thus Onsomeoccasions,theinterferencewassopronouncedthat
the effective area observed will be 3o.8 × 4o.5 centered at someof themodulesof theradarantennahadtobeturned
G48.4−1.4. off. There was some interference due to lightning on most
AsdiscussedearlierRFIisamajorfactorindetermining days.External man made RFI was seen to be verylow.
the success of observations at this frequency. To minimize
man-made interference, observations were to be made dur-
ing night time. G48.4−1.4 could be observed at night with 5 RESULTS
the MST radar during July-August. We conducted our ob-
servations from 30 July to 02 August 2003 between 21:30 Thefinalspectraareinunitsof TTsLys,whereTListhelinean-
and 00:30 hourslocal time. tennatemperatureandTsysisthesystemtemperature.The
While observing, we radiated a tone at frequency 52.6 system temperature is estimated as follows. The sky back-
MHz to make sure that the receiver and data acquisition groundradiation(=26930K)towardstheobservedposition
system were functioning as expected and to confirm that is obtained after scaling the measured values at 34.5 MHz
the frequency switching was taking place as desired. The and using a spectral index of −2.7 (Dwarakanath & Udaya
radiated frequency was chosen to lie outside the frequency Shankar1990,Salter&Brown1988).Themeasuredreceiver
range where the OHline and theRLs are expected. temperature of the radar facility is ∼ 4630K (Damle et al.
1992).Thusthetotalsystemtemperatureis∼31560K.Dur-
ingourobservationsinterferenceduetolightingwaspresent,
which would have increased the system temperature. How-
4 DATA ANALYSIS
ever, this increase in system temperature is not significant
WedevelopedsoftwareinC/C++andAIPS++(Astronom- since the duration of data affected by lightning is less than
icalImageProcessingSystem)toreducethedata.Thedata a few percent of the total observing time. The line antenna
analysis starts with the extraction of the marker, which is temperatures obtained from the spectra are converted to
inserted along with the data, to check for any loss of data. unitsoffluxdensitySL usingtheequation 12SLAeff =kTL,
If a loss of data was detected, the 2 × 640 KBytes of data whereA istheeffectivearea oftheantenna.Takinginto
eff
correspondingtothetwofrequencysettingswerediscarded. account the fact that some of the modules were turned off
The2-bitquantizeddataarethenassignedvoltagevaluesof during the observations (see Section 4), we estimated A
eff
−3,−1,+1or+3asrecommendedfromthehardwarespec- is ∼ 9060 m2. This effective area is consistent with that
ification of thePPR.A 4096 point FFT of thesevoltages is obtained from the calibration observation done on Virgo A
takentoobtaina2048pointpowerspectrum.Thefrequency (3C274).
resolutionofthespectrumis807.5Hzwhichcorrespondsto
∼4.6kms−1.Thesepowerspectrawereaveragedforabout1
sec.ThespectraobtainedwhenLO1wassetto47.8948MHz 5.1 53 MHz OH line
(SLO1)andthoseobtainedwith48.0MHz(SLO2)wereaver- TherestfrequencyoftheOHlineis53.1595MHz.Thefinal
agedseparately.Bothaveragedspectrawerecarefullyexam-
spectrum obtained near this frequency range is shown in
ined for interference. Spectra badly affected by interference Fig.4.Nolinewasdetectedtoa3σlevelof4×10−3 inunits
werediscarded.Spectrashowing interferencein afew chan- of TL .Thiscorrespondstoanupperlimitonfluxof39Jy.
nels were edited using a channel weighting scheme. In this Tsys
Assumingatypicalangularsizeof∼100masforOHmaser
scheme, the channels affected by interference were given a
emission (Hoffman et al. 2003), the brightness temperature
weight of zero (Roshi & Anantharamaiah 2000). Theband-
pass calibrated spectra were obtained as SLO1−SLO2. The of 53 MHz OHmaser line is < 2 × 1012 K.Comparing this
SLO2 temperature with the estimated thermal line temperature
calibrated spectrum along with the channel weights were
(see Section 2), we rule out maser amplification factor > a
weightedaveragedtoobtainthefinalspectrum.Thismethod
few times 1012.
resultedinslightlydifferentintegrationtimesforeachchan-
nel.Themaximumdifferenceinintegrationtimeinthefinal
spectrum was less than 10 percent.
5.2 Carbon Recombination Lines
An average spectrum for each day’s observation was
obtained without shifting the spectra in velocity since the Fiverecombinationlinesofcarbonarepresentwithintheob-
correction for the antenna motion relative to the Local servedbandwidth(seeFig.5).TheyaretheC501α(52.1576
Standard of Rest (LSR) during one observing run (∼ 2.5 MHz),C500α(52.4709MHz),C499α(52.7867MHz),C498α
53 MHz OH line 5
Figure 4. Spectrum near 53.16 MHz obtained towards Figure 6. Spectrum obtained towards G48.4−1.4 by averaging
G48.4−1.4.OHlinewasnotdetectedtoa3σlevelof4×10−3 in the spectra corresponding to five carbon RL transitions near 53
unitsof TL ,whichcorrespondstoanupperlimitonfluxdensity MHz. The observed RL transitions are C501α(52.1576 MHz),
of39Jy.Tsys C500α (52.4709 MHz), C499α (52.7867 MHz), C498α (53.1050
MHz) and C497α (53.4259 MHz). Carbon RL was not detected
to a 3σ upper limit of 2 × 10−3 in units of TL , which corre-
Tsys
spondstoanupperlimitonthefluxdensityof20Jy.
electron densities in the range 0.03 – 0.3 cm−3 and path
52.15 52.47 52.79 53.11 53.15 53.43 lengths between 0.1 – 170 pc can produce the observed
carbon RL intensity at different frequencies (Kantharia &
Anantharamaiah 2001). The observations at 53 MHz to-
wards G48.4−1.4 along with data at other frequencies can
be used to constrain the physical properties of the carbon
C501αC500α C499α C498α OH C497α
Power ltiencetefdoramti3n2g7rMegHiozntoiwnatrhdissGd5ir0e.0ct+io0n.0. wCiatrhbaon2oRbLeawma.sLdinee-
was not detected towards G47.8+0.0 at 327 MHz implying
that the line forming region is less than 4o in extent. The
observation towards G50.0+0.0 has about 1o overlap with
theregionobservedat53MHz.NoRLwasdetectedat34.5
MHz towards G50.0+0.0. This observation has an angular
Frequency resolutionof21′×25o.Combiningthesedata,wefoundthat
carbon line forming region with physical properties in the
range determined by Kantharia & Anantharamaiah (2001)
is consistent with all the three observations. Observations
Figure5.SchematicrepresentationofthepositioningoftheOH with similar angular resolutions are needed to better con-
lineandcarbonRLswithintheobservedbandwidth. strain thephysical properties.
(53.1050 MHz) and C497α (53.4259 MHz) transitions. To
6 CONCLUSION AND FUTURE
improve the signal to noise ratio, we averaged the spectral
POSSIBILITIES
values over the frequency range where these lines are ex-
pected.ThefinalspectrumthusobtainedisshowninFig.6. Inthispaper,wehavepresentedobservationsmadenear53
No line feature was detected to a 3 σ level of 2 × 10−3 in MHz with the aim of detecting the 53.2 MHz transition of
units of TL , which corresponds to an upper limit on the theOHradical.ThisOHlineisduetothetransitionbetween
Tsys
fluxdensity of 20 Jy. the hyperfine levels in the same λ doublet state of lowest
ExtensivesurveysofcarbonRLsfromthegalacticplane energy.TheobservationswereconductedusingtheNational
have been made earlier near 34.5 and 327 MHz (Kantharia MSTRadarFacilityatGadanki.NoOHlinewasdetectedto
& Anantharamaiah 2001, Roshi & Anantharamaiah 2000). a 3σ upper limit of 39 Jy. Within the observed bandwidth,
These observations were used to determine the physical five carbon RLs were present. We have not detected the
properties of the carbon line forming region. Partially ion- carbon lines to a 3σ limit of 20 Jy.
izedregions withelectron temperaturebetween20–300K, External man-made interference is low at Gadanki,
6 S. M. Menon et. al.
which is very encouraging for future attempts to search for SalterC.J.,BrownR.L.,1988,inGalacticandExtragalac-
the OH line in other positions in the Galactic plane. How- tic Radio Astronomy, eds. G. H. Verschuur, K. I. Keller-
ever,currently,amajorlimitation isthelackoftrackingfa- mann, Springer–Verlag, Berlin, p.1
cility for the MST radar antenna. A tracking facility would Turner, B. E., 1979, A&AS,37, 1
allowustosearchforOHlineatotherpositionsintheGalac- Wardle, M., Yusuf-Zadeh,F., 2002, Science, 296, 2350
tic plane in a reasonable observing time. Weaver, H., 1963 in Interstellar Ionised Hydrogen, ed. Y.
Terzian, W. A.Benjamin Inc., NewYork, 644
Weinreb, S.et al., 1963, Nature, 200, 829
ACKNOWLEDGMENTS
We gratefully acknowledge the help of S. Gurushant of Ra-
man Research Institute for helping us set up the PPR as
well as for his help with the observations. We thank Prof.
N. Udaya Shankar for extending the help of the Radio As-
tronomy Lab at RRI for this project. We are grateful to T.
Prabu, C. Vinutha, B. S. Girish and H. N. Nagaraj of Ra-
man Research Institute, Bangalore for their valuable help
in setting up the PPR for our observations. The PPR was
developed by Prof. A. A. Deshpande and team at RRI. We
arethankfulfordiscussionswithProf.A.A.Deshpandethat
led to the initiation of this project. We are indebted to G.
Sarabagopalan of RRI for his generous help in contacting
NMRFand in planningtheobservations. Weare extremely
grateful to Prof. D. Narayana Rao, Director, NMRF for
speedy approval of our observing proposal. His enthusias-
ticsupportwas instrumentalintheefficient conductofthis
project.ItisapleasuretothankS.Kalesha,S.Babuandall
the staff at NMRF, Gadanki for their help with the obser-
vations and a memorable stay at Gadanki. SMM and DAR
are grateful to NMRF for providing the observing time at
theMSTRadarandhospitality.SMMwishestothankRRI
forhospitalityaswellasuseofresourcesduringtheconduct
of this project. He also wishes to thank Sudhakara G., R.
Samanth, M. Raghuprem and P. C. Madhuraj of Manipal
Institute of Technology, Manipal for taking over some aca-
demic responsibilities so that the entire vacation could be
devoted to this project. We thank the anonymous referee
for his helpful suggestions for the continuation of the work
presented in thepaper.
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