Second Generation Surveys For Brown Dwarfs

The development of array detectors has permitted deep, wide-field surveys at near-infrared wavelengths. These arrays offer two advantages first, larger areal coverage in a single exposure, with a higher spatial resolution than is possible with aperture photometry and second, simultaneous measurement of the sky background, and therefore more accurate sky subtraction. At optical wavelengths, arrays of large-format CCDs provide both wide-angle coverage and 10 times more sensitivity than the...

The Early Years

William Herschel H10 inaugurated the study of Galactic structure in the late 1770s through his programme of 'star-gaging' - counting the total number of stars that drifted through the 15-arcmin diameter field of view of his 20-foot reflecting telescope1 within a set time, for a grid of reference fields distributed across the celestial sphere. His observations yielded the average surface density of stars brighter than the limiting magnitude of his telescope (about 12th magnitude), for Herschel...

The Fieldstar Mass Function

Investigations of (M) in the general Galactic Field can be placed under two headings statistical determinations, which compute the luminosity function as an intermediate step and more direct analyses, which use the mass-luminosity relationship to estimate individual stellar masses before combining the data to estimate (M). Both approaches are liable to similar systematic uncertainties arising from uncertainties in the MLR. Rebinning luminosity function data can smooth over important features,...

Info

Photometric light curves of AD Leo in the U-band filter from two observatories, together with the soft X-ray and H7 line flux data. The U-band continuum and the H7 line flux have a similar response to the impulsive flare heating, while the X-ray emission is delayed. These observations support the chromospheric evaporation model described in the text. (Data from H4 .) Figure 5.13. Photometric light curves of AD Leo in the U-band filter from two observatories, together with the soft...

Characteristics Of L And T Dwarfs

The most prominent characteristics of the optical spectra of L and T dwarfs are described in Sections 2.2.3 and 2.2.4, respectively, while Section 2.2.6 gives an overview of the main properties at near-infrared wavelengths. The variations in the relative strengths of the stronger features, namely molecular absorption by TiO, VO, CaH, FeH and CrH at optical wavelengths and FeH, H2O, CO and CH4 at infrared wavelengths, provide the basis for the spectral classification schemes outlined by...

Al mfc

A diffraction grating re-distributes the incident light amongst several orders, with the intensity distribution dependent on the shape of the grooves. For a simple, square-wave grating, most of the energy resides in the undispersed, zero-order image. However, the shape of the grooves can be adjusted to concentrate up to 90 of the flux within a relatively narrow range of 9. Following the grating equation, for any given wavelength this places most of the intensity within a specific order. These...

The Brown Dwarf Appearing In Secchi

'It is a capital mistake to theorise before one has data.' Astronomy is an observational science. In disciplines such as chemistry, biology and physics, carefully controlled laboratory experiments can be set up to test the effect of altering one specific variable, such as temperature, pressure or concentration. However, the physical conditions that pervade in most celestial objects are so extreme that there can be no possibility of their being matched in terrestrial laboratories, so direct...

Gj1001am35v

Spitzer LRS 5-15- m spectra of an M dwarf, GJ1001A (LHS 102A), a late-type L dwarf, 2M0255-4700, and the T dwarf binary, e Indi Bab. Broad ammonia absorption at 7.8 and 10.5 p,m is evident in the latter two spectra. (From R9 , courtesy of the Astrophysical Journal.) Figure 2.11. Spitzer LRS 5-15- m spectra of an M dwarf, GJ1001A (LHS 102A), a late-type L dwarf, 2M0255-4700, and the T dwarf binary, e Indi Bab. Broad ammonia absorption at 7.8 and 10.5 p,m is evident in the latter two...

Av T

The system is stable against convection for AVT < 0. Finally, since the convective inequality can be rewritten as a Fk < for stability dr CP where F is the flux and k is the opacity. Thus, a system is unstable against convection if the flux level is high (as in the central regions of intermediate- and high-mass stars) if gravity is low (as in red giant envelopes) if the opacity is high and if CP is high, which is the case where abundant elements are undergoing ionisation. The last two...

M

Extending these calculations to include evolutionary effects, Bll gives the following relationships 27,700t- .324m .827k R088 Kelvin 1.74 x 10-6t-L297m2.04kR35l0 where r is the age in Gyr, M is the mass in M0, and kr is the Rosseland mean opacity (see Chapter 4). These relationships make no allowance for complications such as deuterium burning or the strong wavelength dependence of atmospheric opacities in low-temperature atmospheres, but provide order of magnitude estimates for fully...

Treatment Of Opacity

The description of the opacity is the single most important factor in producing an accurate photospheric model for a low-mass star. The reader may have encountered references to 'grey' atmospheres (computed assuming the opacity is frequency-independent), or atmospheres calculated using the Rosseland mean opacity (also frequency-independent, but weighted at each depth such that the integrated flux is correct at the surface see M3 ). While adequate for some earlier-type stars, and particularly...

Activity Properties Of The Lowestmass Stars And Brown Dwarfs

The extreme faintness of the very low-mass (YLM) stars and brown dwarfs, coupled with the small size of the available samples, has hampered progress in the understanding of their magnetic activity. Even the choice of magnetic diagnostic 5 It should be noted that these are v sin i measurements for the most part we assume that inclination effects are statistically insignificant when using a large, unbiased sample. Figure 5.24. The activity strength is nearly uncorrelated with rotation, showing...

The Formation Of Lowmass Stars

Star formation is a fundamental (perhaps the fundamental) astrophysical process, underlying heavy element production and the overall evolution of galactic systems. As yet, we have only a partial understanding of the various mechanisms which govern the morphological rearrangement of clouds of diffuse gas into self-gravitating spheres in hydrostatic equilibrium. These uncertainties reflect the complexity of the process, particularly dealing with gas magnetohydrodynamics over scales of up to...

A comparison of the ELS and SZ models

These models make different predictions about the expected properties of the halo population. The modified ELS model predicts a radial abundance gradient and a correlation between rotational velocity and abundance in response to the slow, unidirectional collapse. On the other hand, Searle and Zinn predict a dispersion in age- and abundance-dependent properties with radius, with no monotonic abundance gradient and no expectation of a correlation between rotation and abundance. Observations tend...

Metallicity

Known or suspected low-metallicity stars - which are referred to as subdwarfs due to their position below the solar metallicity main sequence in an H-R diagram (see Chapters 2 and 10) - must have their spectral types, metallicities and temperatures determined self-consistently simply applying the relationships from Table 4.1 will lead to an underestimate of the temperature by several hundred degrees. In general, metallicity affects the atmosphere by reducing the opacity at a given height, hence...

Summary

The main aim of this chapter has been to provide an empirical overview of the properties of cool, late-type dwarfs, and to discuss how those properties can be used to illuminate broader issues such as star formation, nucleosynthesis, stellar magnetic fields, Galactic structure and galaxy formation. Defining M dwarfs as main sequence stars with TiO absorption, their properties can be summarised as follows Their spectra are characterised by molecular absorption bands, notably TiO, VO and metal...

K3 K4 K5 K6 K7 Mo M1 M2 M3 M4 M5 M6 M7m8 M9

The H-R diagram for lower-luminosity stars in the Orion Nebula Cluster. Filled symbols indicate stars with spectroscopic observations open symbols have only photometric data (From H10 , courtesy of L. Hillenbrand and the irowom cal owrwal.) Figure 9.17. The H-R diagram for lower-luminosity stars in the Orion Nebula Cluster. Filled symbols indicate stars with spectroscopic observations open symbols have only photometric data (From H10 , courtesy of L. Hillenbrand and the irowom cal...

Quick Tour Of The Wider Solar Neighbourhood

Most analyses of late-type stars and brown dwarfs concentrate, perforce, on the regions of the Galaxy nearest the Sun. Surveys for ultracool dwarfs (late-M, L and T dwarfs) are restricted to distances of only 20-30 parsecs, but the nearest open clusters and young associations lie at greater distances in particular, the nearest intermediate-age open cluster, the Pleiades, lies at a distance of 130 pc, while most of the young star-forming associations targeted by brown dwarf surveys lie at...

Postmain Sequence Evolution And The Death Of The Milky

Low-mass stars have main sequence lifetimes that are orders of magnitude longer than the present age of the Universe (currently estimated as 12-14 Gyr). Little consideration has been given to the later phases of evolution of these objects, perhaps because the prospects of observing an evolved M dwarf are slight. Eventually, however, even 0.1 M0 dwarfs exhaust their central energy cores, cease hydrogen core-burning and evolve off the main sequence. The final stages of their existence - perhaps...

Homework Problems Problem

Arcturus, the brightest star in the constellation Bootes, has co-ordinates (RA 14 h 15m 39.7s, Dec +19 10'57 equinox 2,000). Calculate the expected rise and set times (LST and local time) for this star as viewed from Sydney, Australia, on 15 June 2000. Suppose that the effective western horizon has a zenith distance of 88 -estimate the actual time that Arcturus disappears. Arcturus is one of the nearest stars, with a measured trigonometric parallax of 88.9 milliarcseconds. The proper motions...

Gd 165b

Searches for low-mass companions require observations that optimise the contrast between the primary and secondary in the potential binary system. This goal can be achieved either through high spatial resolution observations, which minimise the point-spread function of the primary star and therefore allow detection of a faint companion at small separation, or by making observations at wavelengths that minimise the flux difference between primary and secondary, and therefore maximise the chances...

Hjd 2450000

A microlensing planet the figure plots the light curve of the OGLE 2003-BLG-235 MOA 2003-BLG-53 lensing event. The long-term curve is due to the lensing star the sharp spikes at HJD 2835 and 2842 are attributed to a planetary companion of that star (courtesy of B15 and the Astrophysical Journal). Section 9.3.3 summarised how microlensing can be used to estimate masses for isolated stars. The same technique can also be used to search for planetary companions if the foreground lens...

The Magnetic Field 521 Dynamo theory

Poloidal Field Lines Pictures

Galileo first observed sunspots in the early 1600s, but it was George Ellery Hale -using the spectroheliograph at Mount Wilson Observatory in the early 1900s - who identified them as regions of strong magnetic field H1 . The modern theory for the production of the field began with the work of Parker P1 , who proposed a dynamo mechanism for generating and sustaining magnetic fields in the convection zone of the Sun. This model and its many subsequent revisions have come to be known as the afi...