High-resolution near-infrared spectra of protostars

We present new high-resolution (R similar or equal to 21,000) near-infrared (lambda = 2 mu m) spectroscopic observations of a sample of Class I and flat-spectrum protostellar objects in the rho Ophiuchi dark cloud. None of the five Class I spectra show CO upsilon = 0-2 absorption features, consistent with high K-band continuum veilings, 4 less than or similar to r(k) less than or similar to 20, and fast stellar rotation, assuming that the underlying protostellar photospheres are of late spectral type, as is suggested by the low luminosities of most of these objects. Two of the flat-spectrum protostellar objects also show no absorption features and are likely to be highly veiled. The remaining two flat-spectrum sources show weak, broad absorptions, which are consistent with an origin in quickly rotating (upsilon sin i approximate to 50 km s(-1)) late-type stellar photospheres, which are also strongly veiled (r(k) similar or equal to 3-4). These observations provide further evidence that: (1) Class I sources are highly veiled at near-infrared wavelengths, confirming previous findings of lower resolution spectroscopic studies, and (2) flat-spectrum protostars rotate more rapidly than classical T Tauri stars (Class II sources), supporting findings from a recent high-resolution spectroscopic study of other flat-spectrum sources in this cloud. In addition our observations are consistent with the high rotation rates derived for two of the Class I protostellar objects in our sample from observations of variable hard X-ray emission obtained with the ASCA satellite. These observations suggest that certain Class I sources can rotate even more rapidly than flat-spectrum protostars, near breakup velocity.