Young stellar objects in the field

Giant molecular cloud complexes span a wide range of environments, with the extremes being typified by the high star density in the massive Orion star formation region centred on the Trapezium cluster, and the more dispersed, low-mass GMCs in the Taurus complex. The former includes more than 3,000 stars, and will probably evolve to a relatively high-density open cluster, perhaps comparable to the Pleiades, while the individual clouds in the latter system are likely to produce no more than a few...

Speckle interferometry

Labeyrie L1 showed that when radiation from a star passes through the Earth's atmosphere, the wavefront passes through regions of different densities (atmosphere cells) which introduce phase fluctuations. Integrating these fluctuations over time leads to the broad seeing disk illustrated in Figure 1.13. If the exposure time is limited to extremely short timescales (< 100 msec) then some phase coherence remains. A single snapshot reveals a series of bright speckles which contain information on...

The method of trigonometric parallaxes

This is the second method devised by Kapteyn, and was adopted subsequently by van Rhijn R15 , R16 , Bok and Macrae B5 , Trumpler and Weaver T6 , Starikova S11 and is even discussed at length by Mihalas and Binney M8 . Given the observed distribution N(m, k) for a set of stars k > -lim, the aim is to correct both for errors due to random observational uncertainties and for systematic effects due to incompleteness in the sample. Consider the number-magnitude-proper motion distribution, N(m,p),...

Sky Surveys

Large-scale surveys, providing imaging and photometric catalogues for substantial fractions of the celestial sphere, are now available at a wide range of wavelengths. Over the last few years, these surveys have become invaluable research tools for investigating the properties of intrinsically rare objects. This section outlines the main characteristics of the surveys that have proven most useful in searches for low-mass stars and brown dwarfs. The first large-scale imaging surveys were based on...

Temperature determinations

Most M dwarfs have gravity log g 5, and near-solar metallicity, making temperature the most important unknown parameter. Finding the temperature from a comparison of models with observations results in a spectral type-temperature calibration (using spectral data), or a colour-temperature calibration (using photometric data). Both calibrations are in common use, and provide

How To Recognise A Brown Dwarf

Brown dwarfs are defined by their mass. Unfortunately, that parameter is rarely measureable in a direct fashion (Section 9.3). A handful of brown dwarf binaries have been identified that may eventually provide direct calibration of the substellar regime, but so far no accurate mass measurements have been obtained. Lacking such data, we have to infer masses by comparing secondary indicators - luminosity and temperature - against theoretical predictions. The theoretical models described in...

Spectral Classification What Are M L And T Dwarfs Early observations

The advent of optical spectroscopy revolutionised astronomy in the mid-nineteenth century. For the first time, astronomers were placed in a position where they could probe the nature of stellar material. This ability was enhanced greatly when photography was used to record stellar spectra for relatively leisurely inspection, rather than relying on fleeting impressions made at dead of night under strained physiological conditions. Hearnshaw H3 ably describes the early development of stellar...

Planet Formation Classical scenarios

Until very recently, theoretical mechanisms for forming planetary systems were constrained by observations of one system - our own. The inherent pitfalls in dealing with what Wetherill W7 terms 'statistics of one' were well appreciated by planetary scientists and cosmogonists. Nonetheless, models for the formation of our Solar System came to represent the standard paradigm for planetary formation. The main characteristics of the Solar System are first, a significant change in the mean...

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The upper panel plots mass-Mv relationships predicted by intermediate-abundance models from D4 Fe H -1.3, dotted line , A2 m H -1.5, dashed line and B2 m H -1.5, long-dashed line . The results are in good agreement. The lower panel compares the first derivatives from those relationships to halo Mv . Figure 10.17. The upper panel plots mass-Mv relationships predicted by intermediate-abundance models from D4 Fe H -1.3, dotted line , A2 m H -1.5, dashed line and B2 m H -1.5,...