\begin{table}%t1
\caption{\label{TabSources}Properties of background sources, and rms noise levels of the spectra.}
%\centerline
{\small
\begin{tabular}{l @{\hspace{0.3cm}} l @{\hspace{0.3cm}} l @{\hspace{0.3cm}} l
    @{\hspace{0.3cm}} l @{\hspace{0.3cm}} l @{\hspace{0.3cm}} c  @{\hspace{0.3cm}} c @{\hspace{0.3cm}} c @{\hspace{0.3cm}} c  @{\hspace{0.3cm}} c}
\hline\hline\noalign{\smallskip}
Source      & RA(J2000)   & Dec (J2000) & $l$ & $b$ & $D$\tablefootmark{a} & 
$\sigma_l/T_{\rm c}$ \tablefootmark{b} & $\sigma_l/T_{\rm c}$ \tablefootmark{b} & $\sigma_l/T_{\rm c}$
    \tablefootmark{b} & $\sigma_l/T_{\rm c}$ \tablefootmark{b} & $\sigma_l/T_{\rm c}$ \tablefootmark{b} \\
            & (h) (m) (s) & ($^{\circ}$) ($'$) ($''$) &  ($^{\circ}$) & ($^{\circ}$) & (kpc) &
HCO$^{+}$ (0--1) & HCO$^{+}$ (1--2) & HNC (0--1) & HCN (0--1) & CN (0--1)  \\
\hline
G05.88-0.39 & 18 00 30.4 & $-24$ 04 00   & 05.88 & $-0.39$ & 3.8  &
0.021       &            &             &               &       \\
G08.67-0.36 & 18 06 18.9 & $-21$ 37 35   & 08.67 & $-0.36$ & 4.8  & 
0.138       &            &             &               &       \\
G10.62-0.38 & 18 10 28.7 & $-19$ 55 50   & 10.62 & $-0.38$ & 4.8  & 
0.049       & 0.206      & 0.043       & 0.035         & 0.064 \\
G34.3+0.1   & 18 53 18.7 & +01 14 58   & 34.26 & +0.15 & 3.8  & 
0.117       & 0.328      & 0.038       & 0.036         & 0.154  \\
W49N        & 19 10 13.2 & +09 06 12   & 43.17 & +0.01 & 11.5 & 
0.015       & 0.072      & 0.020       & 0.021         & 0.031 \\
W51         & 19 23 43.9 & +14 30 30.5 & 49.49 & $-0.39$ & 7.0  & 
0.022       & 0.089      & 0.043       & 0.034         & 0.070 \\
\hline
\end{tabular}}
\tablefoot{\tablefoottext{a}{Source distance from  Fish \etal (\cite{Fish2003}) who resolve the kinematic
  distance ambiguity. Errors are around 1.0~kpc.}
\tablefoottext{b}{$\sigma_l/T_{\rm c}$ is the rms noise divided by the continuum
  intensity of the spectra, at a frequency resolution of 40~kHz.}}
\end{table}