Context : Studies of the formation and evolution of young stars and their disks rely on knowledge of the stellar parameters of the young stars .
The derivation of these parameters is commonly based on comparison with photospheric template spectra .
Furthermore , chromospheric emission in young active stars impacts the measurement of mass accretion rates , a key quantity for studying disk evolution .

Aims : Here we derive stellar properties of low-mass ( M _ { \star } \lesssim 2 M _ { \odot } ) pre-main sequence stars without disks , which represent ideal photospheric templates for studies of young stars .
We also use these spectra to constrain the impact of chromospheric emission on the measurements of mass accretion rates .
The spectra are reduced , flux-calibrated , and corrected for telluric absorption , and are made available to the community .

Methods : We derive the spectral type for our targets by analyzing the photospheric molecular features present in their VLT/X-Shooter spectra by means of spectral indices and comparison of the relative strength of photospheric absorption features .
We also measure effective temperature , gravity , projected rotational velocity , and radial velocity from our spectra by fitting them with synthetic spectra with the ROTFIT tool .
The targets have negligible extinction ( A _ { V } < 0.5 mag ) and spectral type from G5 to K6 , and from M6.5 to M8 .
They thus complement the library of photospheric templates presented by \citet Manara13a .
We perform synthetic photometry on the spectra to derive the typical colors of young stars in different filters .
We measure the luminosity of the emission lines present in the spectra and estimate the noise due to chromospheric emission in the measurements of accretion luminosity in accreting stars .

Results : We provide a calibration of the photospheric colors of young pre-main sequence stars as a function of their spectral type in a set of standard broad-band optical and near-infrared filters .
The logarithm of the noise on the accretion luminosity normalized to the stellar luminosity is roughly constant and equal to \sim - 2.3 for targets with masses larger than 1 solar mass , and decreases with decreasing temperatures for lower-mass stars .
For stars with masses of \sim 1.5 M _ { \odot } and ages of \sim 1-5 Myr , the chromospheric noise converts to a limit of measurable mass accretion rates of \sim 3 \cdot 10 ^ { -10 } M _ { \odot } /yr .
The limit on the mass accretion rate set by the chromospheric noise is of the order of the lowest measured values of mass accretion rates in Class II objects .

Conclusions :