Hydrogen recombination emission lines commonly observed in accreting young
stellar objects represent a powerful tracer for the gas conditions in the
circumstellar structures. Here we perform a study of the HI decrements and line
profiles, from the Balmer and Paschen lines detected in the X-Shooter spectra
of a homogeneous sample of 36 T Tauri stars in Lupus, the accretion and stellar
properties of which were already derived in a previous work. We aim to obtain
information on the gas physical conditions to derive a consistent picture of
the HI emission mechanisms in pre-main sequence low-mass stars. We have
empirically classified the sources based on their HI line profiles and
decrements. We identified four Balmer decrement types (classified as 1, 2, 3,
and 4) and three Paschen decrement types (A, B, and C), characterised by
different shapes. We first discussed the connection between the decrement types
and the source properties and then compared the observed decrements with
predictions from recently published local line excitation models. One third of
the objects show lines with narrow symmetric profiles, and present similar
Balmer and Paschen decrements (straight decrements, types 2 and A). Lines in
these sources are consistent with optically thin emission from gas with
hydrogen densities of order 10^9 cm^-3 and 5000<T<15000 K. These objects are
associated with low mass accretion rates. Type 4 (L-shaped) Balmer and type B
Paschen decrements are found in conjunction with very wide line profiles and
are characteristic of strong accretors, with optically thick emission from
high-density gas (log n_H > 11 cm^-3). Type 1 (curved) Balmer decrements are
observed only in three sub-luminous sources viewed edge-on, so we speculate
that these are actually reddened type 2 decrements. About 20% of the objects
present type 3 Balmer decrements (bumpy), which cannot be reproduced with
current models.Comment: 29 pages, accepted by A&
