Near IR spectra at low ( R \sim 800 ) and medium ( R \sim 9000 ) resolution , obtained with ISAAC at VLT , have been used to pose constraints on the evolutionary state and accretion properties of a sample of five embedded YSOs located in the R CrA core .
This sample includes three Class I sources ( HH100 IR , IRS2 and IRS5 ) , and two sources with NIR excesses ( IRS6 and IRS3 ) .
IRS5 and IRS6 have been discovered to be binaries with a separation between the two components of 97 and 78 AU , respectively .
Absorption lines , typical of late-type photospheres , have been detected in the medium resolution spectra of all the observed targets , including HH100 IR and IRS2 which have high values of infrared continuum veiling ( r _ { k } = 6 and 3 , respectively ) .
These two sources also present low resolution spectra rich of emission lines ( HI , CO and plenty of other permitted lines from neutral atoms ) likely originating in the disk-star-wind connected regions .
Among the features observed in HH100 IR and IRS2 , Na I at 2.205 \mu m and CO at 2.3 \mu m , which are more commonly used for stellar classification , are detected in emission instead of absorption .
Several strong photospheric lines , which lie around 2.12 and 2.23 \mu m and whose ratio is sensitive to both effective temperature and gravity , are proposed as independent diagnostic tools for this type of sources .
We derived spectral types , veiling and stellar luminosity of the five observed sources , which in turn have been used to infer their mass ( ranging between 0.3-1.2 M _ { \odot } ) and age ( between 10 ^ { 5 } and 10 ^ { 6 } yr ) adopting pre-main sequence evolutionary tracks .
We find that in HH100 IR and IRS2 most of the bolometric luminosity is due to accretion ( L _ { acc } / L _ { bol } \sim 0.8 and 0.6 respectively ) , while the other three investigated sources , including the Class I object IRS5a , present a low accretion activity ( L _ { acc } / L _ { bol } < 0.2 ) .
Mass accretion rates of the order of 2 10 ^ { -6 } and 3 10 ^ { -7 } M _ { \odot } yr ^ { -1 } are derived for HH100 IR and IRS2 , respectively , i.e .
higher by an order of magnitude with respect to the average values derived in T Tauri stars .
We observe a general correlation between the accretion luminosity , the IR veiling and the emission line activity of the sources .
In particular , we find that the correlation between L _ { acc } and L _ { Br \gamma } , previously reported for optical T Tauri stars , can be extended also to the embedded sources , up to at least one order of magnitude larger line luminosity .
A correlation between the accretion activity and the spectral energy distribution slope is recognizable but with the notable exception of IRS5a .
Our analysis therefore shows how the definition of the evolutionary stage of deeply embedded YSOs by means of IR colors needs to be more carefully refined .