UNIVERSITY of CALIFORNIA Los Angeles Characterizing Low-Mass Stars and Brown Dwarfs and Upgrading NIRSPEC a Dissertation Submitt
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UNIVERSITY OF CALIFORNIA Los Angeles Characterizing Low-mass Stars and Brown Dwarfs and Upgrading NIRSPEC A dissertation submitted in partial satisfaction of the requirements for the degree Doctor of Philosophy in Astronomy by Emily Catherine Martin 2018 c Copyright by Emily Catherine Martin 2018 ABSTRACT OF THE DISSERTATION Characterizing Low-mass Stars and Brown Dwarfs and Upgrading NIRSPEC by Emily Catherine Martin Doctor of Philosophy in Astronomy University of California, Los Angeles, 2018 Professor Ian S. McLean, Chair This dissertation combines near-infrared spectroscopic and astrometric analysis of low-mass stars and brown dwarfs with instrumentation work to upgrade the NIRSPEC spectrometer for the Keck II Telescope. The scientific goals of my thesis are to discover and characterize the physical properties of brown dwarfs, the lowest-mass (<0.08 MSun) products of the star formation process. These relatively cold objects (Teff< 2500K, compared to TSun ∼5800K) emit the bulk of their light in the infrared. Project I of my thesis used near-infrared spec- troscopy from NIRSPEC to study the surface gravities of 228 low-mass stars and brown dwarfs in the NIRSPEC Brown Dwarf Spectroscopic Survey (Martin et al., 2017). Project II utilizes imaging data from the Spitzer Space Telescope to study the coldest and lowest-mass brown dwarfs. I measure distances to 22 late-T and Y dwarfs and use those distances to measure absolute physical properties. Project III encompasses my work to upgrade the NIR- SPEC instrument and compliments my scientific research interests. My work on the upgrade includes project management, infrared detector characterization and testing, optical design of the new slit-viewing camera, electronics design, and mechanical testing and prototyping (see Martin et al., 2014, 2016). ii The dissertation of Emily Catherine Martin is approved. Michael P. Fitzgerald J. Davy Kirkpatrick Edward L. Wright Edward Donald Young Ian S. McLean, Committee Chair University of California, Los Angeles 2018 iii To my parents, whose love and support for me extends to the edge of the universe and back. iv TABLE OF CONTENTS 1 Introduction :::::::::::::::::::::::::::::::::::::: 1 1.1 Brown Dwarfs . .3 1.1.1 NIRSPEC Brown Dwarf Spectroscopic Survey . .4 1.1.2 Parallaxes of brown dwarfs . .6 1.2 NIRSPEC Upgrade . .6 2 Surface gravities for 228 M, L, and T dwarfs in the NIRSPEC Brown Dwarf Spectroscopic Survey :::::::::::::::::::::::::::::: 8 2.1 Introduction . .8 2.1.1 Surface Gravity as an Age Indicator . .9 2.1.2 The NIRSPEC Brown Dwarf Spectroscopic Survey . 10 2.2 Sample . 12 2.2.1 Observations . 13 2.2.2 Data Reduction . 13 2.3 Surface Gravity: Methods and Results . 14 2.3.1 Equivalent Widths . 14 2.3.2 FeHJ Index . 18 2.3.3 Gravity Scores . 19 2.3.4 Radial Velocity . 21 2.3.5 Excluded Objects . 22 2.4 Discussion . 23 2.4.1 Comparison to Allers & Liu (2013) . 23 2.4.2 Overall Trends . 23 v 2.4.3 Comparison of Objects of Known Age . 26 2.4.4 Potentially Young Objects . 30 2.4.5 Determining Ages . 33 2.4.6 The Future for Surface Gravity Analysis . 37 2.5 Summary . 38 2.6 Acknowledgements . 39 2.7 Chapter 2 Appendix . 62 3 Y dwarf Trigonometric Parallaxes from the Spitzer Space Telescope :: 68 3.1 Introduction . 68 3.2 Sample . 74 3.3 Photometric and Spectroscopic Follow-up . 75 3.3.1 Ground-based photometry with Palomar/WIRC . 75 3.3.2 Ground-based Spectroscopy with Keck/NIRSPEC . 76 3.3.3 New late-T and Y dwarfs and updated spectral types . 78 3.4 Spitzer Astrometric Follow-Up . 80 3.4.1 Observations . 80 3.4.2 Astrometric and Photometric Data Reductions . 84 3.5 Astrometric Analysis . 85 3.5.1 Coordinate Re-Registration . 85 3.5.2 Positional Uncertainties . 88 3.5.3 Astrometric Solutions . 89 3.6 Results . 102 3.6.1 Spectrophotometric and Photometric Distances for New Discoveries . 102 3.6.2 Comparison to Literature . 102 vi 3.6.3 Comparison to Leggett et al. 2017 . 108 3.6.4 Color Magnitude Diagrams . 113 3.6.5 Absolute Magnitude vs SpT . 115 3.7 Discussion . 119 3.7.1 Not all Y dwarfs are created equal . 119 3.8 Summary . 122 4 NIRSPEC Upgrade for the Keck II Telescope :::::::::::::::: 123 4.1 Introduction . 123 4.2 NIRSPEC Instrument: Current Design and Planned Upgrade . 124 4.2.1 Current Design . 124 4.2.2 Design limitations . 127 4.2.3 Planned Upgrade . 128 4.3 Spectrometer detector upgrade . 131 4.3.1 Optical modeling of the SPEC detector . 131 4.3.2 Replacement Detector Head Prototyping and Initial Testing . 134 4.4 SCAM Upgrade . 134 4.4.1 Planned Upgrade . 134 4.4.2 SCAM Opto-Mechanical Design Constraints . 136 4.4.3 SCAM Upgraded Optical Design . 138 4.5 Laser Frequency Comb Tests . 147 4.5.1 Yi et al. (2016) . 147 4.5.2 Suh et al. (submitted) . 150 4.6 Summary . 151 5 Summary ::::::::::::::::::::::::::::::::::::::: 153 vii LIST OF FIGURES 2.1 Number of targets versus spectral type for all 228 objects in the BDSS. Shading represents gravity classifications, as defined by A13 and as determined in this paper. Red shading represents targets with a gravity classification of VL-G. These objects are likely very young. Green shaded regions denote targets with INT-G classification, indicating youth (∼30{200 Myr). Blue targets have FLD-G gravity classifications, and are generally older than ∼200 Myr. Objects cooler than type L7 cannot be gravity typed by A13's methods and are shown in grey. 12 2.2 Three example J-band spectra of spectral type L3 objects from the BDSS. Each spectrum represents a different gravity type: low gravity (2MASS 2208+2921), intermediate gravity (2MASS 1726+1538), and high gravity (2MASS 1300+1912). Major absorption features in the J-band are also labeled. Light grey shaded regions denote the locations used to calculate K I EWs and the FeHJ index. Dark grey regions denote locations of the pseudo-continua used in our calculations. The K I line at 1.2437 µm is marked, but not shaded. This line was not used to determine gravity types because of contamination from FeH absorption at ∼1.24 µm. .......................................... 15 viii 2.3 K I pseudo equivalent width vs spectral type for all M, L, and T dwarfs in the BDSS for which EWs can be measured. Field dwarfs are shown in black and binaries and subdwarfs are shown as grey stars. Uncertainties are calculated using a Monte Carlo technique with 1000 iterations of modulating the flux by the SNR and re-calculating the EW. The K I lines at 1.1692 µm and 1.1778 µm disappear from T dwarf spectra later than ∼T5 and the K I lines at 1.2437 µm and 1.2529 µm are not found in T dwarfs later than spectral type ∼T7. Shaded regions denote differing gravity types as defined by A13. Objects lying within the salmon shaded regions receive a score of \2" (indicating low gravity), objects in the green shaded regions receive a score of \1" (intermediate gravity), and objects within the blue shaded regions receive a score of \0" (“field” or high gravity). These scores are used along with the FeHJ score to compute a median gravity type. VL-G and INT-G designations are not distinguishable for M5 dwarfs for the K I lines at 1.1692 and 1.1778 µm, and gravity types are not designated for dwarfs of spectral type L8 and later. FeH contamination of the 1.2437 µm line results in larger measurement uncertainties as well as a less-distinguishable low- gravity sequence. For this reason, A13 did not determine cutoff values for gravity types for this line. 17 2.4 FeHJ index vs spectral type for all M, L , and T dwarfs in the BDSS. Normal dwarfs are shown in black and binaries and subdwarfs are shown as grey stars. Shading is the same as in Figure 2.3. This index measures the FeH absorption feature at 1.2 µm using the continuum and absorption bands shown in Figure 2.2. FeH is found in late-type M dwarfs and most L dwarfs. Spectral types later than ∼ L8 have atmospheres cool enough to condense this molecule. Index values of ∼1.0 indicate the absorption feature is nearly absent in the spectra of L/T transition objects. This index re-emerges slightly in the mid-type T dwarfs. 19 ix 2.5 Left: comparison of K I EW values between A13 and this paper. Colors indicate the particular line at which the EWs were calculated. The one-to-one line is shown to aid comparison. Right: comparison of FeHJ index values from A13 and this paper. Despite differing instruments and resolving powers, our values are consistent within the uncertainties. 24 2.6 EW vs SpT and FeHJ vs SpT for dwarfs of spectral type M5.5-L0 with known ages. Different shaped symbols represent the methods used to estimate ages, i.e. group membership or an age estimate from a more massive stellar companion. For references, see Table 2.3. Symbols are colored by their known ages as follows: Red symbols have ages < 30 Myr, green symbols are ∼30 { 100 Myr, and blue symbols are >100 Myr. Binaries from Table 2.3 are not shown, nor is the only single object of spectral type later than L0 in our sample, 2MASS 2244+2043 (L6, VL-G, AB Dor). Spectral types have been distributed randomly in each spectral type bin for ease of viewing. 28 2.7 Age vs K I EW at 1.1692 µm, 1.1778 µm, and 1.2529 µm, binned by spectral type.