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 fl at-spectrum protostellar objects in the rho Ophiuchi dark cloud. None of th e five Class I spectra show CO upsilon = 0-2 absorption features, consisten t 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 underly ing protostellar photospheres are of late spectral type, as is suggested by the low luminosities of most of these objects. Two of the flat-spectrum pr otostellar objects also show no absorption features and are likely to be hi ghly veiled. The remaining two flat-spectrum sources show weak, broad absor ptions, which are consistent with an origin in quickly rotating (upsilon si n i approximate to 50 km s(-1)) late-type stellar photospheres, which are a lso strongly veiled (r(k) similar or equal to 3-4). These observations prov ide further evidence that: (1) Class I sources are highly veiled at near-in frared wavelengths, confirming previous findings of lower resolution spectr oscopic studies, and (2) flat-spectrum protostars rotate more rapidly than classical T Tauri stars (Class II sources), supporting findings from a rece nt high-resolution spectroscopic study of other flat-spectrum sources in th is cloud. In addition our observations are consistent with the high rotatio n rates derived for two of the Class I protostellar objects in our sample f rom observations of variable hard X-ray emission obtained with the ASCA sat ellite. These observations suggest that certain Class I sources can rotate even more rapidly than flat-spectrum protostars, near breakup velocity.