observational techniques buttressed by detailed theoretical models. However, only a handful of the direct detections are firmly established as true SMOs, and the only knowledge researchers possess of the indirect candidates are the lower limits to their masses. Recent data from the Hipparcos satellite on one of the systems has, however, set an upper limit to the inclination which firmly establishes the candidate as substellar in mass.

The number of candidates detected so far in the regions of the sky searched suggests that, extrapolated to the entire sky, thousands more are detectable with present instruments. But the detailed study of such objects, including the collection and analysis of spectra, awaits increased sensitivity of detectors and greater availability of large telescopes. Much like Moses, observers are at present restricted to gazing from afar upon—but not yet experiencing—a promising future rich in the detection and study of SMOs.

Spectroscopic Studies of SMOs

Spectral analysis of SMOs is critical not only to understand the physical properties of these objects but also to identify molecules that tightly constrain the atmospheric temperatures and, hence, allow firm assessment of the objects' masses. Key molecules in this regard are methane (CH4) and ammonia (NH3), which become increasingly abundant at cooler atmospheric temperatures at the expense of carbon monoxide (CO) and molecular nitrogen (N 2). The challenge will be to detect small amounts of methane in objects that are close to the edge of the stellar main sequence: spectra extending further into the infrared will be helpful in this regard but challenge current capabilities. Sensitive upper limits on molecular species that are present in cool stars but should be condensed out of the observable atmospheres of lower-mass and hence cooler objects is another spectroscopic test of SMOs membership that further emphasizes the need for high sensitivity.

The direct detection and analysis of spectra of the most massive SMOs have proceeded sufficiently that many observers are endorsing the creation of an additional letter in the traditional stellar classification sequence of main sequence objects: O, B, A, F, G, K, M. Objects near the main sequence edge have spectra qualitatively different from M dwarfs, in that titanium oxide (TiO) and vanadium oxide (VO) are absent, and other molecular lines dramatically perturb the spectrum from blackbody. A proposal to designate SMOs as “L” dwarfs is not simply an exercise in nomenclature but a recognition of the discretely different nature of the spectrum of objects with surface temperatures of 1500 K and lower.



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