\begin{table}%t1
 \caption{\label{tab:mlines}Methanol maser transitions -- maser class, frequencies,
lower state energies, line strengths, and laboratory and
astronomical references.}
 %\centerline
{ \begin{tabular}{rllllll}
 \hline \hline   
\noalign{\smallskip}                               
Class$^a$& Transition,~$J_k{=}^b$& Frequency (MHz)$^c$ & ~$E^d_\ell$ (cm$^{-1}$) &~$S\mu^2$ (D$^2$)
& Laboratory ref.$^e$ & Astronomical ref.$^f$\\
\noalign{\smallskip}                               
\hline 
\noalign{\smallskip}
I?$^g$ &\d$1_1\to1_1A^{\mp}$ &\d\d\d834.267(2)$^h$ & \d11.705 & \d1.2129 & Radford \cite{Radford1972}          & Ball et~al. (\cite{Ball1970})\\
I?$^g$ &\d$3_{1}\to3_{1}A^{\mp}$ & \d\d5005.32079(20)$^h$ & \d19.703 &\d0.4693 & HD73 & Robinson et~al. (1973)\\
II&\d$5_{1}\to6_{0}A^{+}$   &\d\d6668.5192(8)  & \d33.876 & \d5.0768 & BR95                & Menten (1991)\\
 I&\d$9_{-1}\to8_{-2}E$     &\d\d9936.202(4)   & \d76.102 & \d2.7769 & BR95                & Slysh et~al. (\cite{Slysh1993})\\
II&\d$2_{0}\to3_{-1}E$      &\d12~178.597(4)    & \d13.556 & \d1.9848 & BR95                & Batrla et~al. (\cite{Batrla1987})\\
II&\d$2_{1}\to3_{0}E$       &\d19~967.3961(2)   & \d18.803 & \d0.9244 & M~85                 & Wilson et~al. (\cite{Wilson1985})\\
II&\d$9_{2}\to10_{1}A^{+}$  &\d23~121.0242(5)   & \d98.053 & \d3.1467 & M~85                 & Wilson et~al. (\cite{Wilson1984})\\
 I&\d$3_{2}\to3_{1 }E$      &\d24~928.707(7)    & \d24.310 & \d2.8073 & This work           & Barrett et~al. (\cite{Barrett1975})\\
 I&\d$4_{2}\to4_{1}E$       &\d24~933.468(2)    & \d30.764 & \d3.9283 & Gaines et~al. (1974)  & Barrett et~al. (\cite{Barrett1971}  \\
 I&\d$2_{2}\to2_{1}E$       &\d24~934.382(5)    & \d19.469 & \d1.5948 & Gaines et~al. (1974)  & Barrett et~al. (\cite{Barrett1975})\\
 I&\d$5_{2}\to5_{1}E$       &\d24~959.0789(4)   & \d38.833 & \d5.0264 & M~85                 & Barrett et~al. (\cite{Barrett1971})\\
 I&\d$6_{2}\to6_{1}E$       &\d25~018.1225(4)   & \d48.514 & \d6.1287 & M~85                 & Barrett et~al. (\cite{Barrett1971})\\
 I&\d$7_{2}\to7_{1}E$       &\d25~124.8719(4)   & \d59.809 & \d7.2483 & M~85                 & Barrett et~al. (\cite{Barrett1971})\\
 I&\d$8_{2}\to8_{1}E$       &\d25~294.4165(2)   & \d72.717 & \d8.3910 & M~85                 & Barrett et~al. (\cite{Barrett1971})\\
 I&\d$9_{2}\to9_{1}E$       &\d25~541.3979(4)   & \d87.239 & \d9.5570 & M~85                 & Menten et~al. (\cite{Menten1986})\\
 I&$10_{2}\to10_{1}E$       &\d25~878.2661(4)   &  103.373 &  10.7398 & M~85                 & Matsakis et~al. (\cite{Matsakis1980})\\
 I&$12_{2}\to12_{1}E$       &\d26~847.233(50)   &  140.478 &  13.0919 & T95                 & Wilson et~al. (\cite{Wilson1996})\\
 I&$13_{2}\to13_{1}E$       &\d27~472.531(30)   &  161.449 &  14.2057 & T95                 & Wilson et~al. (\cite{Wilson1996})\\
 I&$14_{2}\to14_{1}E$       &\d28~169.462(30)   &  184.032 &  15.2244 & T95                 & Wilson et~al. (\cite{Wilson1996})\\
 I&$15_{2}\to15_{1}E$       &\d28~905.812(30)   &  208.226 &  16.0977 & T95                 & Wilson et~al. (\cite{Wilson1996})\\
II&\d$8_{2}\to9_{1}A^{-}$   &\d28~969.942(50)   & \d83.319 & \d3.0236 & T95                 & Wilson et~al. 1993 \\
 I&$16_{2}\to16_{1}E$       &\d29~636.936(10)   &  234.031 &  16.7716 & T95                 & Wilson et~al. (\cite{Wilson1996})\\
 I&$17_{2}\to17_{1}E$       &\d30~308.034(10)   &  261.445 &  17.1961 & T95                 & Wilson et~al. (\cite{Wilson1996})\\
 I&\d$4_{-1}\to3_{0}E$      &\d36~169.265(30)   & \d18.803 & \d2.5184 & T95                 & Morimoto et~al. (\cite{Morimoto1985})$^\star$ \\
II&\d$7_{-2}\to8_{-1}E$     &\d37~703.700(30)   & \d61.930 & \d2.4051 & T95                 & Haschick et~al. (\cite{Haschick1989})\\
II&\d$6_{2}\to5_{3}A^{+}$   &\d38~293.268(50)   & \d58.813 & \d0.9488 & T95                 & Haschick et~al. (\cite{Haschick1989})\\
II&\d$6_{2}\to5_{3}A^{-}$   &\d38~452.677(50)   & \d58.813 & \d0.9495 & T95                 & Haschick et~al. (\cite{Haschick1989})\\
 I&\d$7_{0}\to6_{1}A^{+}$   &\d44~069.410(10)   & \d43.694 & \d6.1380 & T95                 & Morimoto et~al. (\cite{Morimoto1985})\\
 I&\d$5_{-1}\to4_{0}E$      &\d84~521.169(10)   & \d25.254 & \d3.0830 & This work           & Batrla \& Menten (\cite{Batrla1988})$^\star$ \\
II&\d$7_{2}\to6_{3}A^{-}$   &\d86~615.600(5)    & \d68.493 & \d1.3578 & This work           & Sutton et~al. (\cite{Sutton2001})$^\star$\\
II&\d$7_{2}\to6_{3}A^{+}$   &\d86~902.949(5)    & \d68.493 & \d1.3596 & This work           & Sutton et~al. (\cite{Sutton2001})$^\star$\\
 I&\d$8_{0}\to7_{1}A^{+}$   &\d95~169.463(10)   & \d54.888 & \d7.2211 & This work           & Plambeck \&\ Wright (\cite{Plambeck1988})$^\star$\\
 I&$11_{-1}\to10_{-2}E$     & 104~300.414(7)    &  106.779 & \d3.4141 & This work          & Voronkov et~al. (\cite{Voronkov2004})$^\star$\\
II&\d$3_{1}\to4_{0}A^{+}$   & 107~013.803(5)    & \d16.134 & \d3.0088 & This work           & Val'tts et~al. 1995a$^\star$\\
II&\d$0_{0}\to1_{-1}E$      & 108~893.963(7)    &\d\d5.490 & \d0.9784 & This work           & Val'tts et~al. (\cite{Val1999})$^\star$\\
 I&\d$6_{-1}\to5_{0}E$      & 132~890.692(10)   & \d33.316 & \d3.6871 & T95                 & Slysh et~al. (\cite{Slysh1997})$^\star$\\
I&\d$9_{0}\to8_{1}A^{+}$    & 146~618.794(50)   & \d67.679 & \d8.3288 & T95                 & Menten (1991b)     \\
II&\d$8_{0}\to8_{-1}E$      & 156~488.868(10)   & \d61.930 & \d6.7830 & T95                 & Slysh et~al. (\cite{Slysh1995a})$^\star$\\
II&\d$2_{1}\to3_{0}A^{+}$   & 156~602.413(10)   &\d\d9.681 & \d1.9963 & T95                 & Slysh et~al. 1995a$^\star$\\
II&\d$7_{0}\to7_{-1}E$      & 156~828.533(10)   & \d49.035 & \d6.2749 & T95                 & Slysh et~al. (\cite{Slysh1995a})$^\star$ \\
II&\d$6_{0}\to6_{-1}E$      & 157~048.625(10)   & \d37.749 & \d5.6636 & T95                 & Slysh et~al. (\cite{Slysh1995a})$^\star$\\
II&\d$5_{0}\to5_{-1}E$      & 157~179.017(10)   & \d28.073 & \d4.9589 & T95                 & Slysh et~al. (\cite{Slysh1995a})$^\star$\\
II&\d$4_{0}\to4_{-1}E$      & 157~246.056(10)   & \d20.009 & \d4.1725 & T95                 & Slysh et~al. (\cite{Slysh1995a})$^\star$\\
II&\d$1_{0}\to1_{-1}E$      & 157~270.851(10)   &\d\d5.490 & \d1.4611 & T95                 & Slysh et~al. (\cite{Slysh1995a})$^\star$\\
II&\d$3_{0}\to3_{-1}E$      & 157~272.369(10)   & \d13.556 & \d3.3178 & T95                 & Slysh et~al. (\cite{Slysh1995a})$^\star$\\
II&\d$2_{0}\to2_{-1}E$      & 157~276.058(10)   &\d\d8.717 & \d2.4090 & T95                 & Slysh et~al. (\cite{Slysh1995a})$^\star$\\
 I&\d$8_{-1}\to7_{0}E$      & 229~758.760(50)   & \d54.266 & \d5.0473 & Sastry et~al. (\cite{Sastry1985}  & Slysh et~al. (\cite{Slysh2002})$^\star$\\
\noalign{\smallskip}
 \hline
 \end{tabular}}
\smallskip
$^a$ Gives the maser type. Interstellar methanol masers are either
class~I or to II; see text and Menten (\cite{Menten1991a},b).
$^b$ The parity of~$A$ symmetry transitions is given as superscript.
If the parities of the upper (left) and lower (right) state differ,
the parities are given as upper and lower superscript, respectively.
$^c$ Numbers in parentheses denote the measurement uncertainties
in units of the least significant figures.
$^d$ Lower state level energies are {\it all} relative to the
ground-state ($0^+_0$) level of the~$A$-symmetry species.  The E-type
ground state ($1_{-1}$) level is at a 5.490~cm$^{-1}$ higher energy.
$^e$ HD73 stands for Heuvel \&\ Dymanus (1973); BR95 for Breckenridge \&\
Kukolich (\cite{Breckenridge1995}), M~85 for Mehrotra et~al. (\cite{Mehrotra1985}), and~T95 for Tsunekawa et~al. (\cite{Tsunekawa1995}).
$^f$ Only the first reference reporting \textit{maser} emission
in this line is given.  An asterisk means that this line was earlier
detected in thermal emission or absorption.
See Lovas (\cite{Lovas2004}) for relevant references.
$^g$ See Sect.~4.3. for a discussion of these lines.
$^h$ The~$^1$H hyperfine splitting has been resolved to some extent in
the laboratory. For the~$J = 1 \to 1$ transition, four components, spread
over 19.4~km~s$^{-1}$, have been observed. The frequency in the table
is the one of the strongest component; the intensity weighted average
is 1~kHz lower.
For the~$J = 3 \to 3$ transition, two components, split by~0.19~km~s$^{-1}$, have been observed.
\end{table}