We present radio continuum observations of the planetary nebula ( PN ) IRAS 17347-3139 ( one of the only two known to harbour water maser emission ) , made to derive its spectral index and the turnover frequency of the emission .
The spectrum of the source rises in the whole frequency range sampled , from 2.4 to 24.9 GHz , although the spectral index seems to decrease at the highest frequencies ( 0.79 \pm 0.04 between 4.3 and 8.9 GHz , and 0.64 \pm 0.06 between 16.1 and 24.9 GHz ) .
This suggests a turnover frequency around 20 GHz ( which is unusual among PNe , whose radio emission usually becomes optically thin at frequencies < 10 GHz ) , and a relatively high emission measure ( 1.5 \times 10 ^ { 9 } cm ^ { -6 } pc ) .
The radio continuum emission has increased by a factor of \simeq 1.26 at 8.4 GHz in 13 years , which can be explained as expansion of the ionized region by a factor of \simeq 1.12 in radius with a dynamical age of \simeq 120 yr and at an expansion velocity of \simeq 5 - 40 km s ^ { -1 } .
These radio continuum characteristics , together with the presence of water maser emission and a strong optical extinction suggest that IRAS 17347-3139 is one of the youngest PNe known , with a relatively massive progenitor star .