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Register a new userCarbon Monoxide in low-mass dwarf stars
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We compare high resolution infrared observations of the CO 3-1 bands in the 2.297-2.310 micron region of M dwarfs and one L dwarf with theoretical expectations. We find a good match between the observational and synthetic spectra throughout the 2000-3500K temperature regime investigated. Nonetheless, for the 2500-3500 K temperature range the temperatures that we derive from synthetic spectral fits are higher than expected from more empirical methods by several hundred K. In order to reconcile our findings with the empirical temperature scale it is necessary to invoke warming of the model atmosphere used to construct the synthetic spectra. We consider that the most likely reason for the back-warming is missing high temperature opacity due to water vapour. We compare the water vapour opacity of the Partridge & Schwenke (1997) line list used for the model atmosphere with the output from a preliminary calculation by Barber & Tennyson (2004). While the Partridge & Schwenke line list is a reasonable spectroscopic match for the new line list at 2000 K, by 4000 K it is missing around 25% of the water vapour opacity. We thus consider that the offset between empirical and synthetic temperature scales is explained by the lack of hot water vapour used for computation of the synthetic spectra. For our coolest objects with temperatures below 2500 K we find best fits when using synthetic spectra which include dust emission. Our spectra also allow us to constrain the rotational velocities of our sources, and these velocities are consistent with the broad trend of rotational velocities increasing from M to L.
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Yakiv V. Pavlenko Main Astronomicaln Observatory of Academy of Sciences of Ukraine
, Golosiiv woods
2002
We compare the observational and theoretical spectra of the $Delta v$ = 2 CO bands in a range of M dwarfs. We investigate the dependence of theoretical spectra on effective temperatures as well as carbon abundance. In general we find that the synthetic CO bands fit the observed data extremely well and are excellent diagnostics. In particular the synthetic spectra reasonably match observations and the best fit temperatures are similar to those found by empirical methods. We also examine the CDC isotopic ratio. We find that fundamental $^{13}$CO bands around 2.345 and 2.375 $mu$m are good discriminators for the CDC ratio in M dwarfs. The 2.375 $mu$m is more useful because it doesnt suffer such serious contamination by water vapour transitions. Our current dataset does not quite have the wavelength coverage to perform a reliable determination of the CDC ratio in M dwarfs. For this we recommend observing the region 2.31--2.40 $mu$m at a resolution of better than 1000. Alternatively the observational problems of contamination by water vapour at 2.345 $mu$m maybe solved by observing at resolutions of around 50000. We also investigated the possibility of using the $Delta v$ = 1 CO bands around 4.5 $mu$m. We find that the contamination due to water vapour is even more of a problem at these wavelengths.
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Extremely metal-poor galaxies with metallicity below 10% of the solar value in the local universe are the best analogues to investigating the interstellar medium at a quasi-primitive environment in the early universe. In spite of the ongoing formation of stars in these galaxies, the presence of molecular gas (which is known to provide the material reservoir for star formation in galaxies, such as our Milky Way) remains unclear. Here, we report the detection of carbon monoxide (CO), the primary tracer of molecular gas, in a galaxy with 7% solar metallicity, with additional detections in two galaxies at higher metallicities. Such detections offer direct evidence for the existence of molecular gas in these galaxies that contain few metals. Using archived infrared data, it is shown that the molecular gas mass per CO luminosity at extremely low metallicity is approximately one-thousand times the Milky Way value.
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