\documentclass[journal]{rmaa}


\title{Compact Radio Sources Apparently Associated\\ with Extended Galactic Sources}

\author{Alfonso Trejo and Luis F. Rodr\'iguez\affil{Centro de Radioastronom\'ia y Astrof\'isica\\ 
Universidad Nacional Aut\'onoma de M\'exico, Morelia, Michoac\'an, Mexico}} 


\fulladdresses{

\item Alfonso Trejo and Luis F. Rodr\'iguez: Centro de Radiostronom\'ia y Astrof\'isica, Universidad Nacional Aut\'onoma de M\'exico, Apdo. 
Postal 3-72, (Xangari), 58089 Morelia, Michoac\'an, Mexico (a.trejo,~l.rodriguez@crya.unam.mx).} 


\listofauthors{A.~Trejo \& L.~F.~Rodr\'iguez}

\indexauthor{Trejo, A.}
\indexauthor{Rodr\'iguez, L.~F.}


\shortauthor{Trejo \& Rodr\'iguez}

\shorttitle{HI Absorption toward Compact Radio Sources}

\SetVolume{46} 
\SetYear{2010}
\ReceivedDate{2010 January 12} 
\AcceptedDate{2010 July 26} 


\resumen{Reportamos observaciones hechas con el VLA de la l\'inea de 21 cm del HI hacia dos fuentes compactas que podr\'ian estar 
asociadas con fuentes gal\'acticas extendidas. En el caso de la nebulosa planetaria PHR 1735-333 observamos HI en absorci\'on hacia una 
fuente de radio no t\'ermica recientemente descubierta en la regi\'on, la cual se propuso estaba f\'isicamente asociada con la nebulosa planetaria. 
Sin embargo, el an\'alisis del espectro de HI en absorci\'on sugiere una distancia mayor para esta fuente no t\'ermica. En el caso del candidato 
a remanente de supernova SNR G3.8+0.3 obtuvimos espectros de HI en absorci\'on hacia ella y hacia la fuente compacta de radio localizada en 
su centro. Concluimos que SNR G3.8+0.3 es m\'as distante que la fuente compacta y que por lo tanto no est\'an asociadas f\'isicamente.}
 

\abstract{We report VLA radio observations of the 21 cm HI line toward two compact radio sources that could be associated with extended 
Galactic sources. In the case of the planetary nebula PHR 1735-333 we observed HI absorption against a non-thermal radio source recently 
discovered in the region, which was proposed to be physically associated with the planetary nebula. However, from the analysis of the HI 
absorption spectrum, we suggest a larger distance for this non-thermal source. In the case of the supernova remnant candidate SNR G3.8+0.3 
we obtained HI absorption spectra towards it and towards a compact radio source located at its center. We conclude that SNR G3.8+0.3 is more 
distant than the compact radio source and that they are not physically associated.}
      

\addkeyword{ISM: individual (SNR G3.8+0.3)} 
\addkeyword{ISM: supernova remnants}
\addkeyword{planetary nebulae: individual (PHR 1735-333)}
\addkeyword{radio lines: ISM}



\begin{document}

\maketitle

\section{Introduction}

Compact radio sources sometimes appear associated in the plane of the sky with Galactic regions of extended emission. This association, if 
real, can be very important, since the compact source may be tracing the exciting source of the nebulosity or its interaction with other 
sources of energy such as the wind from a central star. On the other hand, we may just be dealing with a fortuitous line-of-sight alignment, 
with no real physical association. We have started a program of observations of such apparent associations using several techniques to 
determine the distance to the objects (Trejo \& Rodr\'iguez 2006, 2008, 2010), in an attempt to support or refute the association.

In this paper we present two cases of such an apparent association. One is the planetary nebula PHR 1735-333, and the other is the supernova 
remnant candidate SNR G3.8+0.3. In both cases we have been able to obtain HI absorption spectra that allow a determination of the distances 
to the sources and a better understanding of their nature. In both cases we favor a fortuitous line-of-sight alignment and propose that the 
compact source and the extended region of emission are not physically associated. 


\section{PHR 1735-333}

The OH/IR star OH 354.88-0.54 (V1018 Sco) was detected as a mid-infrared point source in the Revised Air Force Geophysics Laboratory 
Rocket Sky Survey of Price \& Walker (1976) as source AFGL 5356. It was independently discovered as a strong 1612-MHz maser source 
during a survey of part of the Galactic plane (Caswell et al. 1981). Recently, Cohen, Parker, \& Chapman (2005) discovered a planetary 
nebula around this star using data from the Anglo-Australian Observatory/United Kingdom Schmidt Telescope H$\alpha$ Survey of the Southern 
Milky Way (Parker et al. 2005). This planetary nebula is named PHR 1735-333 in the MASH Planetary Nebula Catalog of Parker et al. (2006). 
There are several studies of this star involving different wavelengths. Green et al. (1999) found continuum emission associated with 
OH 354.88-0.54 in the first Molonglo Galactic Plane Survey at 843~MHz. The detected source was unresolved with a beam of approximately 
$1'$. Cohen et al. (2006) made a study of this source and its environment by means of 3, 6, 13 and 20~cm continuum and 1.3~cm line observations. 
These observations were taken with ATCA. Previously, only OH and SiO masers were reported in association with this source (e.g., 
Cohen et al. 2005; Nyman, Hall, \& Bertre 1993), and Cohen et al. (2006) clearly detected water maser emission as well.

Cohen et al. (2006) found two non-thermal radio sources inside the planetary nebula (see Figure~\ref{opt}). Source A is relatively strong 
($\sim 8$~mJy at 20~cm) and partially resolved, while source B is weaker, unresolved and has no detection at 20~cm. The spectral indices 
of both sources are similar, $-0.81 \pm 0.01$ and $-0.95 \pm 0.11$, for A and B respectively. The sources lie at about the same projected 
angular distance from the central star. At 13~cm Cohen et al. found weak emission (2$\sigma$) apparently connecting both sources, but it is 
unclear if such extended emission is real. If confirmed, this connection would strengthen the possible association of sources A and B with 
the nebula.

It is well known that non-thermal emission can be found in massive stars (e.g., Abbott, Bieging, \& Churchwell 1984; Persi et al. 1985). 
This non-thermal emission is believed to originate by embedded shocks in the stellar winds, as a faster component catches up with 
previous slower ejecta (White 1985). In a source such as PHR 1735-333, it can also be expected that the fast wind characteristic of the 
planetary nebula stage reaches and shocks the slow AGB wind, creating non-thermal emission.


%%%%% FIGURA 1

\begin{figure}[!t]
\centering
\includegraphics[width=0.99\columnwidth]{atrejofig1.eps}
\caption{Optical H$\alpha$ image shown in grey, taken from the SuperCOSMOS database. The contours trace 
the 6~cm radio emission and show both non-thermal sources. The contours are 0.3, 0.5, 0.8, 1.1, 1.4, 1.6, 
1.9, and 2.2~mJy~beam$^{-1}$. The names of the sources (A \& B) are as in Cohen et al. (2006). The cross 
marks the position of the star OH 354.88-0.54 and the white circle shows the approximate size of the PN 
candidate PHR 1735-333, which is visible in the optical image. This image is taken from Cohen et al. (2006).}
\label{opt}
\vspace{0.3cm}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


In their study of PHR 1735-333, Cohen et al. (2006) proposed that the non-thermal radio sources were directly related to the nebula and 
not just line-of-sight alignments, since the probabilities of finding a single source at those positions are 1/500 and 1/200, for 
the observed 6~cm and 20~cm flux densities. The probability of finding two sources, if they are independent, will be much smaller 
($\sim 10^{-5}$). The synchrotron emission would be generated, as they proposed, by the interaction between the slow AGB wind and the more 
recent fast wind from the nucleus of the PN. Since the presence of this type of interaction at relatively large distances from the star 
has not been reported in other sources and could be potentially very important, we tested the association of the non-thermal source A 
with the planetary nebula via HI absorption observations.


\subsection{Observations}

We made observations of the 21~cm HI line using the NRAO\footnote{The National Radio Astronomy Observatory is a facility of the National 
Science Foundation operated under cooperative agreement by Associated Universities, Inc.} Very Large Array in its B configuration. A total 
of 26 antennas were used at the L band. We observed on 2008 March 6 under project AR661 with a total on-source time of about 1.5 hours. 
We used a bandwidth of 3.125~MHz with the 2IF mode (both circular polarizations). A total of 64 channels, each of 48.8~kHz wide, were used, 
which gave us a velocity resolution of 10.3~km~s$^{-1}$.

We tried different weighting schemes in the task IMAGR, with ROBUST$=$0 (Briggs 1995) resulting as the best compromise between angular 
resolution and sensitivity. In order to avoid the emission of the Galactic plane we constructed images and spectra excluding baselines shorter 
that 0.7~K$\lambda$ (1~K$\lambda=10^3$ wavelenghts), suppressing features more extended than $\sim 5'$. The data were processed in the 
standard manner, following the procedures of the AIPS Cookbook\footnote{\url{http://www.aips.nrao.edu/cook.html}.}. The beam had a half 
power full width of $34\rlap.{''}3\times 12\rlap.{''}8$, PA$=3\rlap.^{\circ} 5$, with a positional accuracy of $\sim 3''$.


%%%%% FIGURA 2

\begin{figure}[!t]
\centering
\includegraphics[width=0.99\columnwidth]{atrejofig2.eps}
\caption{Upper panel: Spectrum of the non-thermal source A seen in projection toward PHR 1735-333. The 
spectrum shows absorption at $-140$~km~s$^{-1}$, which points to a farther location than that of the 
planetary nebula. Lower panel: Spectrum of a relatively strong Galactic source near the non-thermal 
source toward PHR 1735-333. It can be seen that the $-140$~km~s$^{-1}$ feature is not present in this 
last spectrum.}
\label{exa}
\vspace{0.3cm}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%%%%%% FIGURA 3

\begin{figure}[!t]
\centering
\includegraphics[width=0.99\columnwidth]{atrejofig3.eps}
\caption{LSR radial velocity as a function of distance to the Sun in the direction of PHR 1735-333. 
The positive values of the LSR radial velocity come from gas beyond the solar circle. The arrow marks 
the distance to the Sun where the edge of the Galaxy is located.}
\label{rot}
\vspace{0.3cm}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%%%%%% TABLA 1

\begin{table*}[!t]
\vspace{0.1cm}
\centering
\caption{Sources in the field of PHR 1735-333 for which\\ HI spectra were extracted}
\small
\begin{tabular}{lccccc}
\toprule
&  & \multicolumn{2}{c}{Position} &  & 20~cm Flux  \\
\cmidrule{3-4} 
Source &  &  $\alpha$(J2000) & $\delta$(J2000) &  & Density (mJy) \\ 
\midrule
OH 354.88-0.54 A &  & 17 35 01.734 & $-$33 33 36.36 &  & $~2.7\pm0.5$ \\
Galactic source  &  & 17 35 00.070 & $-$33 24 17.10 &  & $20.5\pm0.5$ \\
\bottomrule
\label{tab:1}
\end{tabular}
%\vspace{0.2cm}
\end{table*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


\subsection{Results and discussion}

The main goal of these observations was to obtain an HI absorption spectrum of the brighter non-thermal source (source A) seen toward 
the planetary nebula PHR 1735-333, with the purpose of constraining its distance and testing its association with the PN. As the target 
source turned out to be weak at the epoch of observation, we got a spectrum with modest signal to noise ratio (see top panel of 
Figure~\ref{exa}), with two absorption features. Since the continuum and the line are observed simultaneously and are perfectly aligned, 
we only need to determine the peak of the continuum emission and there take the spectra. The values for the HI absorption features 
are $S_{\rm L}/S_{\rm C}=-1.22 \pm 0.15$ (7.9$\sigma$), $V_\mathrm{LSR}=-0.6\pm1.4$~km~s$^{-1}$ and $\Delta V=19.5\pm2.8$~km~s$^{-1}$ for one, and 
$S_{\rm L}/S_{\rm C}=-0.68 \pm 0.15$ (4.5$\sigma$), $V_\mathrm{LSR}=-138.3\pm2.7$~km~s$^{-1}$ and $\Delta V=15.0\pm4.6$~km~s$^{-1}$ for the other, 
where $S_L/S_C$ is the normalized ratio between line and continuum flux densities, $V_\mathrm{LSR}$ is the radial velocity of the component 
referred to the local standard of rest, and $\Delta V$ stands for the full width at half minimum of the fitting. In the case of the 
Galactic source near PHR 1735-333 (see lower panel of Figure~2), the HI absorption feature has $S_{\rm L}/S_{\rm C}=-0.75 \pm 0.04$ (18.8$\sigma$), 
$V_\mathrm{LSR}=+1.9\pm0.8$~km~s$^{-1}$ and $\Delta V=28.7\pm1.9$~km~s$^{-1}$. We considered as real only features above 4$\sigma$. 
Figure~\ref{exa} shows the spectra and the fit superposed on it. The derived line widths are broadened by the instrumental resolution 
(10.3~km~s$^{-1}$) and the deconvolved values are $\sim 17$ and $\sim 11$~km~s$^{-1}$ for the two absorption components shown in the upper
panel of Figure~2, and $\sim 27$~km~s$^{-1}$ for the absorption shown in the lower panel of Figure~2. The HI absorption at a $V_\mathrm{LSR}$ near 
0~km~s$^{-1}$ is attributed to nearby gas.

On the other hand, considering the rotation curve of the Galaxy (Brand \& Blitz 1993) and assuming that the HI disk of the Galaxy 
has an outer radius of 25~kpc (Dickey et al. 2009), we show in Figure~\ref{rot} the LSR velocity versus distance to the Sun for a 
source at the Galactic latitude and longitude of PHR 1735-333. From this figure we can see that if the non-thermal source A is associated 
with the PN, we expect HI absorption components with LSR radial velocities only in the 0 to $-11$~km~s$^{-1}$ range. In contrast, 
if the radio source is extragalactic we expect possible HI absorption components from 0 to $-180$~km~s$^{-1}$. In addition, HI absorption 
at positive velocities (0 to +13~km~s$^{-1}$) may be detected from gas outside the solar circle and located on the other side of the Galaxy. 

Then, the absorption feature at approximately $-140$~km~s$^{-1}$ suggests that the gas responsible for that absorption is located 
at more than 8~kpc, according to Figure~\ref{rot}. We then propose that this non-thermal source is not related to the planetary 
nebula, which is located at a smaller distance of 3.2~kpc (Cohen et al. 2005). It should be noted, however, that the detection of the 
$-140$~km~s$^{-1}$ absorption feature is at the level of only 4.5$\sigma$ level and that additional studies are required to be 
fully confident that the source A is more distant than the planetary nebula. 

The position and flux density of this non thermal source is given in Table~1. For comparison, we plot in the lower panel of Figure~\ref{exa} 
an example of a spectrum of another source present in the field observed. There is very little information on this source. It is 
detected in the NRAO VLA Sky Survey (Condon et al. 1998) with a 1.4~GHz flux density of $21.6 \pm 0.6$~mJy, in good agreement with the value 
found by us (see Table~1). Since it shows HI absorption only near 0~km~s$^{-1}$, we will assume it is a nearby Galactic source. We can see 
that the spectrum towards this source does not present the absorption feature at $\sim -140$~km~s$^{-1}$.

Finally, Cohen et al. (2006) reported a 20~cm flux density of $7.68 \pm 0.18$~mJy for the non-thermal source A, from their 2005 
March 24 observations. We found this flux to be of only 2.7 mJy (see Table~1) in our 2008 March 6 observations. As we noted before, 
source A could be an extragalactic object, such as a quasar. Then, a change in the flux density by a factor of $\sim 3$ is not 
unexpected for this type of sources. However, it should be noted that changes have also been observed in some planetary nebulae 
(G\'omez et al. 2005; Zijlstra, van Hoof, \& Perley 2008). This decrease in flux density was unfortunate, since it reduced the signal-to-noise 
ratio of the HI absorption spectrum toward source A. It will be interesting to study from the theoretical point of view if flux 
density changes of this order and timescale can be produced by the interaction of two winds. 


%%%%%% FIGURA 4

\begin{figure}[!t]
\centering
\includegraphics[width=0.99\columnwidth]{atrejofig4.ps}
\caption{VLA contour image of the continuum emission at 1.5~GHz of the supernova remnant candidate 
SNR G3.8+0.3. Contours are 3, 4, 5, 6, 8, 10, 12, 15, 20, 25, and 30 times 0.5~mJy~beam$^{-1}$. The 
image is not corrected for the primary beam response (the observations were made centered at the 
position of the phase calibrator 1751--253 at $\alpha(2000)=17^h~ 51^m~51\rlap.^s265; 
\delta(2000)=-25^{\circ}~ 23'~59\rlap.{''}80$) and the eastern edge of the shell appears weaker in 
comparison to the western edge than it really is. The compact source GPSR 3.826+0.386 is labeled in 
the figure. The rectangle indicates the region over which the HI absorption spectrum of the SNR candidate 
was obtained. The half-power contour of the synthesized beam ($48\rlap.{''}4 \times 30\rlap.{''}8$; 
PA=$3^{\circ}$) is shown in the bottom left corner of the image.}
\label{fig:snr candidate}
\vspace{0.23cm}
\end{figure}       

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%\vspace{0.2cm}

\section{SNR Candidate G3.8+0.3}

The radio continuum source SNR G3.8+0.3 is a supernova remnant candidate that was detected by Gray (1994) in the Molonglo Observatory 
Synthesis Telescope (MOST) Galactic center survey made at 843~MHz. As noted by Gray, this source shows an incomplete ring almost perfectly 
centered on a compact source (see Figure~4). The supernova remnant candidate has been studied by Bhatnagar (2002) at 327~MHz, who determined 
a spectral index of $\alpha=-0.6 \pm 0.1$ ($S_\nu \propto \nu^\alpha$) between 327 and 843~MHz. This result implies a synchrotron origin 
for the emission and strongly supports a supernova remnant nature for the extended radio source. Case \& Bhattacharya (1998) used a radio 
surface brightness-to-diameter ($\Sigma$-D) relation to estimate a distance of 9.6~kpc to SNR G3.8+0.3, while Stupar et al. (2007) obtain 
a distance of 7.7~kpc from a different relation. We adopt an estimate of $8.7 \pm 1.0$~kpc for the distance to the SNR. Examination of 
available H$\alpha$ survey imaging data such as for the AAO/UKST H-alpha survey (Parker et al. 2005) reveals no evidence of optical emission 
from the SNR candidate. There are no known X-ray sources or pulsars associated with this SNR candidate.
 
The compact radio source at the center of SNR candidate G3.8+0.3 is associated with the source IRAS~17498--2526. This far-infrared source 
was proposed to be a possible planetary nebula by Preite-Mart\'inez (1988) and Pottasch et al. (1988) on the basis of its IRAS colors. 
However, it should be noted that this association is uncertain. The radio source is listed in the catalog of small-diameter radio sources in the 
Galactic plane of Zoonematkermani et al. (1990) as GPSR 3.826+0.386 and we use this nomenclature to refer to it. These authors determine at 
1.4~GHz a flux density of 45~mJy and an angular size of $2\rlap.{''}3$ for the source. In their 5~GHz VLA survey of the Galactic plane, 
Becker et al. (1994) report the source with a flux density of 50.7~mJy and an angular size of $3\rlap.{''}1$. The flat spectrum observed 
between 1.4 and 5.0~GHz is consistent with an optically-thin free-free source such as a planetary nebula or an HII region. The {\sl a priori} 
probability of finding a $\sim 50$~mJy source at 5~GHz in a region with angular dimensions of $2' \times 2'$ (that approximately define 
the center of the SNR remnant candidate) is only $\sim 0.0003$ (Fomalont et al. 1991). The position of GPSR 3.826+0.386 is 
$\alpha(2000)=17^h~52^m~59\rlap.^s28; \delta(2000)=-25^{\circ}~27{'}~24\rlap.{''}7$. The images of the AAO/UKST H$\alpha$ survey (Parker 
et al. 2005) show that this source is a strong and compact H$\alpha$ emitter (see Figure~5) and that optical spectroscopy should be pursued. 


%%%%%% FIGURA 5

\begin{figure}[!t]
\vspace{0.1cm}
\centering
\includegraphics[width=0.85\columnwidth]{atrejofig5.ps}
\caption{$4 \times 4$ arcmin extracts of the AAO/UKST H$\alpha$ SuperCOSMOS survey (Parker et al. 2005) 
around the compact source GPSR 3.826+0.386. Top: H$\alpha$ emission; center: short red emission; bottom: 
H$\alpha$ minus short red emission. Only GPSR 3.826+0.386 remains detectable in the bottom image, indicating 
it is a strong H$\alpha$ emitter. The weaker features in this last image are the result of an imperfect 
subtraction.}
\label{fig:cosmos images}
%\vspace{0.2cm}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


\subsection{Observations}

SNR G3.8+0.3 has practically not been observed with the VLA. Fortunately, it is located at $\sim 15'$ from the well known VLA phase 
calibrator 1751--253 that has been extensively observed. We have then used data centered on 1751--253 to study SNR G3.8+0.3. Although 
the latter source is located at a position where the primary beam response at 20~cm of the 1751--253 observations is $\sim 0.5$ of that at 
the center, the observations can be used taking this factor into account.

In Figure~4 we show a 1.5~GHz image made with archive VLA data taken on 1984 May 31 as part of project AE32. The VLA was then in the C 
configuration and the absolute amplitude calibrator was 1331+305. The bootstrapped flux density of 1751--253 was $1.24 \pm 0.01$~Jy. 
The image shows the incomplete shell morphology of SNR G3.8+0.3 and the central location of GPSR 3.826+0.386.

The HI observations, made in 1989 October 21 as part of project AL201, were taken from the archive of the Very Large Array. The VLA was 
then in the C configuration, providing an angular resolution of about $40''$ for images made with natural weighting. The absolute amplitude 
calibrator was 1331+305 (with an adopted flux density of 14.73~Jy at 1.4~GHz). The bootstrapped flux density of 1751--253 was $1.02 \pm 0.01$~Jy. 
The data were edited and calibrated using the software package Astronomical Image Processing System (AIPS) of NRAO. The spectra consisted 
of two windows, one centered below in frequency (1419.406~MHz) and the other above in frequency (1421.406~MHz) of the HI rest frequency 
(1420.406~MHz). Each window had 127 channels of 24.4~kHz (5.2~km~s$^{-1}$) each. In order to avoid the emission of the Galactic plane we made 
images and spectra excluding baselines shorter that 0.5~K$\lambda$, supressing features more extended than $\sim 7'$. We analyzed separately 
each window and overlapped them to produce a single spectrum for the sources studied. 


\subsection{Results and discussion}

In Figure~6 we show the HI absorption spectra toward three sources in the field: the phase calibrator 1751--253, the brightest part 
of the western shell of SNR G3.8+0.3, and the compact source GPSR 3.826+0.386. 


%%%%% FIGURA 6 

\begin{figure}[!t]
\centerline{\includegraphics[width=0.86\columnwidth]{atrejofig6a.eps}}
\vspace{0.3cm}
\centerline{\includegraphics[width=0.86\columnwidth]{atrejofig6b.eps}}
\vspace{0.3cm}
\centerline{\includegraphics[width=0.86\columnwidth]{atrejofig6c.eps}}
\vspace{0.35cm}
\caption{21 cm H I absorption spectra for the three sources discussed in the text, plotted as a function 
of LSR radial velocity. The measured spectra were divided by the continuum level $(-\tau_v)$, where $\tau_v$ 
is the opacity as a function of velocity.}
\label{hispectra}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


To help in the discussion, we show in Figure~7 the LSR velocity versus distance to the Sun for a source at the Galactic latitude 
and longitude of GPSR 3.826+0.386, obtained as for the case of PHR 1735-333. The phase calibrator $1751-253$ is an extragalactic 
source and, as expected, its HI absorption spectrum confirms this. The spectrum shows strong absorption near 0~km~s$^{-1}$, as 
expected since most of the gas in the direction of this region appears near the LSR radial velocity. In addition, the absorption 
spectrum shows a clear feature at +160~km~s$^{-1}$ that is coming from gas close to the subcentral point. Finally, this spectrum 
also shows absorption at $-$41~km~s$^{-1}$, that is coming from beyond the solar circle. Our HI spectrum is in good agreement with 
that presented by Dickey et al. (1983), although these authors do not cover a sufficiently large velocity interval to include the 
+160~km~s$^{-1}$ feature.


%%%%% FIGURA 7

\begin{figure}[!t]
\centering
\includegraphics[width=0.98\columnwidth]{atrejofig7.eps}
\caption{LSR radial velocity as a function of distance to the Sun in the direction of GPSR 3.826+0.386. 
The negative values of the LSR radial velocity come from gas beyond the solar circle. The arrow marks 
the distance to the Sun where the edge of the Galaxy is located.}
\label{rot2}
\vspace{0.3cm}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%5


The HI spectra of SNR G3.8+0.3 and GPSR 3.826+0.386 both show absorption around 0~km~s$^{-1}$. However, the spectrum of SNR G3.8+0.3 
clearly shows an absorption at +170~km~s$^{-1}$ which places this supernova remnant candidate beyond the subcentral point, at 8.5~kpc. 
This lower limit for the distance is consistent with the value estimated from the results of Case \& Bhattacharya (1998) and Stupar 
et al. (2007), $8.7 \pm 1.0$~kpc. In contrast, GPSR 3.826+0.386 does not show this absorption, suggesting it is closer to us than 
the subcentral point. This upper limit for the distance is consistent with the value of 1.7~kpc estimated by Preite-Mart\'inez (1988), 
from a modified Shklovski (statistical) method applied to all the planetary nebulae in his sample. We then conclude that the supernova 
remnant candidate and the compact source are most likely not associated.

\vspace{-0.15cm}

\section{Conclusions}

We made HI line radio observations toward the planetary nebula PHR 1735-333 and analyzed archive data of the supernova remnant 
candidate SNR G3.8+0.3. Both sources are associated in the plane of the sky with compact radio sources that could be physically 
related to them. Our study shows that the distance to the non-thermal radio source recently found in apparent association with 
the planetary nebula PHR 1735-333 is at least two times larger than that of the nebula. We conclude that the radio source is most 
probably unrelated to the Galactic planetary nebula and we suggest an extragalactic nature for it. However, there are several arguments 
that favor the association as real: (i) the low probability of detection of two sources within the PN shell (see above), (ii) the 
possible link between them at 13 cm, (iii) the fact that the radio sources are equidistant from the compact central source, and 
(iv) the low signal-to-noise ratio of the HI detection at source A. Additional observations are clearly required.

In the case of the supernova remnant candidate SNR G3.8+0.3, we conclude that it is located beyond the subcentral point, at more than 
8.5~kpc from the Sun, while the compact source GPSR 3.826+0.386 is located closer than the subcentral point and thus the two sources 
are not physically associated.

\vspace{0.1cm}

\acknowledgments

We thank the referee for many suggestions and for pointing out that the source GPSR 3.826+0.386 is a bright H$\alpha$ source. 
AT is supported by a Conacyt scholarship. LFR acknowledges the support of DGAPA, Universidad Nacional Aut\'onoma de M\'exico, 
and of Conacyt (Mexico). This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France.


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