The Size, Structure, and Variability of Late-Type Stars Measured ...

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85 Chapter 5 Spectroscopy and ISI Measurements of Spectral Features An incredible amount of information can be obtained through analysis of the spectra of stars. The variations in intensity with wavelength probes the structure of those outer regions of a star in which the optical depth can vary between transparency and opaqueness. Low-resolution spectrometry or bolometry translates into knowledge of the color-temperature and dust shell structure of a star. Observed spectral lines can pinpoint some of the species surrounding stars and their strengths can yield their column densities. The differences between line strengths of like constituents offer clues to the temperature structure around the star. Finally, the redshifts and profile of the lines can be modelled to retrieve information on the velocities of the source gases. When attempting to perform accurate diameter measurements of the photospheric continuum of stars, spectral measurements are an invaluable tool to ascertain which layers are being studied. We saw in Section 3.3 that observing at a wavelength known to contain strong and/or many spectral lines will likely affect a diameter measurement a great deal. With this in mind, an analysis of the accuracy of a set of diameter measurements would benefit from clear spectral measurements of the star being measured. In addition, using a detailed spectrum, it should be possible to perform interferometry designed to probe the structure of a specific constituent of a stellar atmosphere.
86 5.1 General Results of 11 µm Spectroscopy High resolution 11 µm spectra of o Cet, R Leo, χ Cyg, α Ori, and α Her were recorded at the IRTF telescope with the TEXES instrument. 1 Spectral lines identified as H 2 O have been conclusively observed in o Cet, R Leo, α Her, and α Ori. Other unidentified lines appear to be present in some of these spectra. Some of the spectra are plotted in Figure 5.1. The figure shows measured flux as a function of wavenumber near 11 µm (solid curve, left axis). The spectral lines are ∼ 0.03 cm −1 wide corresponding to a velocity distribution about 10 km/s wide. For each spectrum, theoretically calculated H 2 O lines corresponding to some temperature and redshift are illustrated below (vertical bars, right axis). These line positions and strengths were calculated using the HITRAN/HITEMP database described in Rothman et al. (1992) [87] and references therein. The temperatures and redshifts were chosen by eye to best correlate with the spectra shown. It is clear that the largest lines in R Leo and o Cet are due to water vapor. It also appears that H 2 O is present in α Her and α Ori as well. (Not all of the data are shown here, and the rest of the data further support the existence of water in α Ori, which is not clear from Figure 5.1. Water also has been observed in α Ori with a temperature ∼1500 K and column density ∼10 20 cm −2 by Tsuji (2000) [99].) H 2 O is not apparent in χ Cyg. The observed spectral lines are manifested as absorption in α Her, and both emission and absorption in the other stars. In o Cet, a characteristic strong emission peak is seen at a wavenumber approximately 0.033 cm −1 higher than the absorption line (corresponding to a relative blue shift of 10.96 km/s.) We can interpret these observed spectral lines and ascertain some information on the temperatures of their sources by comparing the line depths and the relative line strengths with a theoretical prediction. We will construct a simple model consisting of an infinitely thin spherical shell of gas of radius, R gas , in LTE at some temperature, T gas , and having some optical thickness, τ gas , along a radial path length. The shell surrounds an opaque stellar blackbody of temperature, T ∗ . In this case, the absorption (or emission) line depth and the normalized intensity distribution will depend on the three parameters, R gas , τ gas , and the intensity ratio, B ν (T gas )/B ν (T ∗ ). Figure 5.2 shows the predicted spectral line absorption or emission depth as a function of those three parameters. (The fractional line 1 The observations were carried out by John Lacy and Matt Richter, and a description of the instrument can be found in Lacy et al. (2002) [58].
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The Size, Structure, and Variabilit
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1 Abstract The Size, Structure, and
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iv 5 Spectroscopy and ISI Measureme
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vi 3.15 Density, Temperature, and C
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viii Acknowledgements Over the cour
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2 to achieve resolution characteris
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4 sources are not coaligned perfect
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6 Intensity Profile Hankel Transfor
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8 Figure 1.3: Possible Effective Ba
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10 6LJQDO'HWHFWRU 6LJQDO'HWHFWRU /D
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1995A&A...304...69P 12 Figure 1.5:
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Figure 1.6: Stellar Evolution of a
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16 such as Mira variables, become u
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18 wavelengths, however, its range
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20 Figure 2.1: ISI Data for o Cet f
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22 Figure 2.3: Uniform Disk, Limb D
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24 2.2 Comparison of ISI Data with
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26 Table 2.3: Diameter Measurements
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28 measurements. χ Cyg and R Leo a
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30 metric flux (from the reference
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32 their surface. Very often, mater
Page 43 and 44: 34 Mira. Finally, variations in tim
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Page 47 and 48: 38 Figure 3.2: Intensity Distributi
Page 49 and 50: 40 Figure 3.4: Ratio of the Best-Fi
Page 51 and 52: Figure 3.5: Synthetic Near-Infrared
Page 53 and 54: 44 Angular Diameter (mas) 60 50 40
Page 55 and 56: 46 wavelengths which would cause th
Page 57 and 58: 48 3.4.1 Static Radiative Transfer
Page 59 and 60: 50 Figure 3.8: Ratio of Photospheri
Page 61 and 62: 52 3.4.2 The Effect of Dynamic Phen
Page 63 and 64: 54 Figure 3.10: “Standard” Bowe
Page 65 and 66: 56 extension is reflected at wavele
Page 67 and 68: 58 photosphere. These results suppo
Page 69 and 70: 60 Figure 3.13: Continuum Opacity o
Page 71 and 72: 62 assuming only continuum sources.
Page 73 and 74: 64 Figure 3.16: Normalized Syntheti
Page 75 and 76: Figure 3.17: Density and Temperatur
Page 77 and 78: 68 Figure 3.19: Apparent Size of H
Page 79 and 80: 70 of Fox and Wood (1985) [31] are
Page 81 and 82: 72 3.5 Explanation of Variations in
Page 83 and 84: 74 Figure 3.21: 2.1 µm Image of 25
Page 85 and 86: 76 at visible wavelengths either by
Page 87 and 88: 78 4.1 Implications of Diameter Mea
Page 89 and 90: 80 11 µm Diameter of o Cet vs. Dat
Page 91 and 92: 82 Figure 4.3: A uniform intensity
Page 93: 84 phase observed in the continuum
Page 97 and 98: 88 depth are the contours plotted a
Page 99 and 100: 90 Figure 5.3: Best Fitting Calcula
Page 101 and 102: 92 of its three parameters. The con
Page 103 and 104: 94 H 2 O on their measured diameter
Page 105 and 106: 96 5.3 Modelling Spectral Features
Page 107 and 108: 98 Figure 5.7: Stellar Intensity Pr
Page 109 and 110: Figure 5.9: Observed H 2 O Spectral
Page 111 and 112: Figure 5.10: Höfner Model Predicte
Page 113 and 114: 104 30 km/s causes the spectrum to
Page 115 and 116: Figure 5.13: o Cet Spectra in Obser
Page 117 and 118: 108 density prediction matches the
Page 119 and 120: 110 Dixon. The infrared angular dia
Page 121 and 122: 112 [29] M. U. Feuchtinger, E. A. D
Page 123 and 124: 114 [50] A. P. Jacob, T. R. Bedding
Page 125 and 126: 116 [70] A. A. Michelson and F. G.
Page 127 and 128: 118 [92] Martin Schwarzschild. Over
Page 129: 120 [114] P. Woitke, D. Krüger, an
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