High Resolution IR Spectroscopic Surveys for Protoplanetary Systems with Silicon Immersion Gratings 1

Transcription

1 High Resolution IR Spectroscopic Surveys for Protoplanetary Systems with Silicon Immersion Gratings 1 Jian Ge, Dan McDavitt, Shane Miller & Abhijit Chakraborty 525 Davey Lab, Department of Astronomy & Astrophysics, Penn State University, University Park, PA The breakthrough of silicon immersion grating technology at Penn State has the ability to revolutionize high-resolution infrared spectroscopy at large ground-based telescopes. Fabrication of high quality silicon grisms and immersion gratings up to 2 inches in dimension has become a routine process thanks to newly developed techniques. Silicon immersion gratings with etched dimensions of ~ 4 inches are being developed at Penn State. This immersion grating will be able to provide diffraction-limited spectral resolution of R = 300,000 at 2.2 micron, or 130,000 at 4.6 micron. To take full advantage of this high dispersing device for high resolution IR spectroscopy at high efficiency, high order adaptive optics is required to fully correct wavefronts distorted by atmospheric turbulence, to reach Strehl ratio of at least ~50%. IR spectroscopy with R > 100,000 opens up new possibilities in investigating the total mass and location of protoplanets through observing absorption lines from the CO fundamental bands at 4.6 microns and other molecular bands formed in the dynamic gaps created by protoplanets. It can also be used to study the density, temperature and composition of the environment where planets form. Large aperture telescopes with low thermal background are essential for ground-based observations to have enough sensitivity for observing thousands of nearby T Tauri stars to study planet formation. The results of protoplanet mass and location distribution will be compared to those of planets obtained from Doppler radial velocity surveys to investigate whether orbital migration and dynamical scattering play a significant role in planet formation and evolution. Future perspectives for developing silicon immersion gratings with sizes larger than 4 inches will also be discussed. Key Words: Protoplanets, silicom immersion grating, adaptive optics, telescope, infrared, T Tauri stars 1. Introduction Two fundamental questions that motivate planet searches are how planets form and how they evolve. These basic issues essentially determine the ubiquity (frequency), composition (gaseous versus solid), and structure (with or without cores) of planets as well as the diversity (period, eccentricity, and inclination distribution) and survivability (probability and time scale for fostering biological evolution) of planetary systems. In the past ten years, about 100 planets have been detected with the Doppler radial velocity (RV) technique (for a current list of systems and their properties see These data show that planets are indeed quite ubiquitous around solar type stars and that their dynamical properties are very diverse. These discoveries of extra-solar planets and the determination of their orbital properties have provided golden opportunities for new breakthroughs in the quest to understand the origin and evolution of planets and planetary systems. However, the RV technique is difficult to apply in the study of planet formation due to the existence of a dusty and massive planetary disk. A new, more promising approach is to detect planet formation via the dynamical impact of the planet on the structure of its parent disk. As a planet forms, tidal interactions between the planet and disk act to clear out a region in the disk (a gap ) surrounding the planet (Lin et al for a review). Definitive, kinematic evidence for gaps can be obtained from high resolution spectroscopic studies of line emission from residual gas in the gap, 1 Send correspondence to Jian Ge, jian@astro.psu.edu, Tel: , Fax:

2 which will appear bright against the weak or absent continuum (Carr & Najita 1998; Carr, Mathieu, & Najita 2001). Among all candidate emission lines, the most promising transitions are the CO emissions in the fundamental band at 4.6 µm. The typical projected disk rotation velocities are ~ 10 km/s at 1 AU. To detect the formation of a 1 M J planet at an orbital distance of 1 AU, high signal-to-noise ratio (S/N 40) is required to detect the emission lines produced by an optically thin residual gas in the gap (Carr & Najita 1998). Only ~ 8m ground-based telescopes currently have the sensitivity for such an investigation with a high resolution IR spectrometer with R ~ 100,000 (Carr & Najita 1998). However, it is extremely difficult to build IR high-resolution spectrometers with a conventionally ruled echelle grating at large telescopes. This is because under seeing-limited conditions the spectral resolving power is coupled to telescope aperture size. A spectral resolution of R = 100,000 requires an R2 echelle (tan θ B = 2) with a ruled area of cm 2 on a 10-m telescope using a 1 arcsec slit. Such gratings and their ancillary optics are so large and expensive that alternative technologies must be sought for the new 10 m class telescopes. Immersion grating technology is a promising one that can increase the spectral resolving power without increasing instrument size. The concept of immersion gratings was introduced in the 1950 s when researchers realized that the resolving power of diffraction gratings can be increased by immersing the grating in a transparent medium of high refractive index (Hulthen 1950a, b; Hulthen et al. 1954). This kind of grating is called an immersion grating. By using an immersion grating, the resolution can be increased by a factor of n (the index of refraction of the medium in which the grating is immersed) as shown in Equation (1) (Dekker 1987). 2nd R = tanθ B. (1) ϕ s D where d is the diameter of the beam, φ s is the projected angular slit width, D is the diameter of the telescope, and θ B is the blaze angle of the grating. Furthermore, by inclining the entrance face of the prism, the beam is widened by an anamorphic magnification power, m = cosθt / cosθi, where θ t and θ i are the angles of transmission and incidence at the prism face, respectively. This magnification transforms Equation (1) into 2nmd R = D tanθ B (Dekker 1987). (2) ϕ s If the anamorphic immersion grating is operated at Brewster s angle, the resolving power is increased to n 2 compared to a regular reflection grating with equal length and blaze angle. It is clear that the gain in spectral resolving power is significant if a material with very high refractive index is used for making the immersion gratings. Silicon immersion gratings are of special interest for increasing dispersion power of IR spectroscopy because silicon has one of the highest refractive indices, n = 3.4, and has excellent transmission between µm in the cryogenic temperatures and the grating grooves can be etched on silicon substrates by certain chemical reagents, e.g. potassium hydroxide (KOH) and tetramethyl ammonium hydroxide (TMAH) (e.g. Tsang & Wang 1975; Ge et al. 2001), which etch the silicon (100) planes hundreds of times faster than the (111) planes to form very sharp and smooth V-shaped grating grooves. The size of the grooves is determined by the initial groove mask size. Hence, very coarse gratings can be fabricated by careful design of grating masks and control of the chemical etching process. Coarse gratings will allow us to build cross-dispersed spectrometers with large continuous wavelength coverage on 2-D detectors to achieve a multiplex advantage in IR spectroscopy (i.e. increase instantaneous wavelength coverage). This new grating technique permits the production of gratings with much coarser grooves than are possible with conventional ruling techniques. Currently no echelle gratings are commercially available with fewer

3 than 23.2 grooves per millimeter (or grooves larger than about 43 µm). Therefore, silicon immersion gratings, especially silicon anamorphic immersion gratings, will meet the challenging demand for high resolution IR spectroscopy at large ground-based telescopes. The gain in spectral resolving power for a silicon anamorphic immersion grating is n 2 ~ 12 times when it is operated at Brewster s incident angle. Figure 1 shows a direct comparison between a silicon anamorphic grating operated at Brewster s incident angle (73.8 ) and a regular reflection grating, demonstrating the potential of this new grating for reducing instrument collimator beam size. Silicon anamorphic immersion grating For the same diffraction-limited resolution (R ~ 73.8º 250,000) at 2.2 µm, a new kind of spectrometer 70.5º with the anamorphic silicon immersion grating 54.7º d = 16 mm 36 mm with a 54.7 blaze angle (natural blaze for Regular reflection echelle grating silicon immersion gratings) requires only a 16 R = 250,000 mm diameter beam, about 12 times smaller than a conventional spectrometer with a regular echelle at the same blaze angle. This can lead to orders of magnitude reduction in size and weight d = 189 mm of the new spectrometer, resulting in much less cost and time to construct IR cryogenic high resolution spectroscopic instruments. Once the very high resolution IR spectroscopy is available at the large ground-based telescopes, hundreds of nearby T Tauri stars can be potentially searched for molecular emission lines associated with the dynamical gaps in the protoplanetary disks. The combination of this young planet search with the Doppler RV surveys for old planets will provide not only a more complete census of the extra-solar planets but also provide clues for understanding the formation and evolution of planetary systems. These studies will provide important information in at least three general areas of extra-solar planetary sciences: 1) the dynamical structure of newly formed planetary systems and their potential for retaining terrestrial planets in habitable zones, 2) the origin and diversity of extra-solar planets, and 3) the evolution of multiple planetary systems. 95 mm 2. Status of Silicon Immersion Grating Development 328 mm Figure 1. Comparison between a silicon anamorphic immersion grating and a conventional reflection grating. Figure 2. The etched silicon wafer gratings with the TMAH process and without AP (left) and with AP dose (middle). On the right, a commercially made fused-silica transmission grating with 10 µm grooves is shown for comparison. Irregularity and surface roughness can be clearly seen in this commercial grating. Previous work has demonstrated the feasibility of making high quality gratings out of silicon wafers using micromachining techniques (Tsang & Wang 1975; Wiedemann et al. 1993; Kuzmenko, Ciarlo & Stevens

4 1994; Graf et al. 1994; Jaffe et al. 1998; Kuzmenko & Ciarlo 1998). However, a true immersion grating, where the etched surface is on the hypotenuse of a prism, requires the processing of thick pieces of silicon. This presents many technical challenges (Wiedemann et al. 1993; Kuzmenko, Ciarlo & Stevens 1994). A breakthrough in developing silicon immersion grating technology was made by a team led by Jian Ge in High quality prototype silicon immersion gratings were successfully fabricated using new micromachining techniques (Ge et al. 1999a,b; 2000, 2001). Instead of using the etched silicon gratings in an immersed reflection mode we operated them in an immersed transmission mode. These devices are called silicon grisms. They can be conveniently evaluated in an IR camera by inserting them at the camera pupil location. A diffraction-limited spectral resolution of R = 4900 was achieved with the best silicon grism in the UC Berkeley IRCAL near-ir camera with a 0.15 arcsec entrance slit and a 5 mm diameter camera pupil in 1999 (Ge et al. 2001). For the best silicon grism, the total integrated scattered light is 8% in the K band, and the grating peak efficiency is 45% (Ge et al. 2001). November 2001 March 2002 June 2002 Figure 3. Atomic Force Microscope scanning of the best etched silicon gratings in November 2001, Marc 2002 and June The RMS surface roughness over 2 µm in randomly chosen grating surfaces are 9 nm, nm and 0.9 nm from the right to the left, respectively. Our main focus over last two years has been on the development of new fabrication processes for reducing grating surface roughness, increasing grating efficiency and increasing etched grating areas. The newly developed etching process with tetramethyl ammonium hydroxide (TMAH), ammonium persulfate (AP) and a thin silicon dioxide mask (~ 100 nm thickness) was able to significantly reduce the RMS roughness. Figure 2 shows scanning electron micrograph (SEM) pictures of etched gratings with and without AP processes, and also a commercially made grating. The grating with the AP process has much smoother grating facets and fewer defects. An atomic force microscope is applied for measuring the RMS surface roughness of etched grating surfaces. The results from November 2001, March 2002, and June 2002 are shown in Figure 3. It is very clear that the grating surface gets much smoother in June In November 2001, the typical RMS surface roughness for the best etched silicon gratings with the TMAH and AP process over ~ 1 µm scale is 9 Figure 4. Cross-section of a HeNe laser at µm, indicating <1% integrated scattered light for the TMAH etched grating. nm. In March 2002, the RMS surface roughness for the best etched gratings is down to 3 nm. In June 2002, the rms roughness is reduced to 0.9 nm. The surface roughness for these gratings is lower than that

5 we achieved with the KOH etching process two years ago (Ge et al. 2001), and it is also lower than that for the HST mirror, 12 nm, indicating super smooth grating surfaces. One of the gratings with rms surface roughness of ~ 15 nm has been evaluated with our optical spectrograph; the total measured integrated scatter at µm is less than 1% (Figure 4). This level of scatter is already a factor of three times better than that of a commercial 23.2 l/mm echelle grating measured by Kuzmenko & Ciarlo (1998). Wavefront quality of the etched gratings has been evaluated by a Zygo interferometer. Figure 5 shows the wavefront over mm 2 etched grating area at µm in reflection. The RMS wavefront distortion is waves, indicating diffraction-limited performance once the grating is operated in both immersed reflection and transmission. Maintaining high wavefront quality is critical for achieving the highest dispersion power provided by silicon immersion gratings. Figure 5. Wavefront map of a silicon grating with mm 2 etched area at µm measured by a Zygo interferometer. This new technique has been applied in fabricating new generation silicon grisms. RMS surface roughness of ~ 10 nm has been achieved. Figure 6 shows four grisms installed in mechnical mounts and ready to be installed in the PI s Penn State near IR Imager and Spectrograph (PIRIS) Figure 6. New generation of silicon grisms made of TMAH plus AP processes are mounted in holders and ready for observing with PIRIS (the left three, with etched areas of ~ 10x10 mm 2 ) and ARIES (the right one, with etched area of 15x15 mm 2 ). 0.2 arcsec Order 80, 1.95 µm R = 2000 Brγ absorption line Order 65, 2.36 µm Figure 7. K band image of a Be star, BD , and its companions with the Lick natural guide AO (left). Cross-dispersed silicon echelle grism spectrum of BD in the K band obtained with the IRCAL camera at the Lick 3m in 2000 (middle). Reduced Brγ absorption line spectrum (right). and also the Arizona Imager and Echelle Spectrograph (ARIES) for the MMT 6.5-m telescope. An earlier generation silicon grism has been successfully used in scientific investigations of young stellar objects (Ge et al. 2001). Figure 7 shows a cross-dispersed silicon echelle grism spectrum of a Be star, BD This is achieved by silicon grisms operated in orders with very coarse grooves, 13.3 l/mm, a

6 factor of two times coarser than commercially available echelles. With a low dispersion cross-disperser, 16 cross-dispersed orders of spectra are packed on the PICNIC array in the IRCAL camera. This demonstrates that new silicon grating technology allows design of gratings to match any two dimensional IR array for continuous large wavelength coverage at high resolution. Data analysis shows that an enormous broad Brγ absorption line with a FWHM of 550 ± 50 km/s is associated with BD (~ 90% of stellar break-up velocity), indicating this star is perhaps undergoing an early phase of rotation breaking and will become a normal Be star (Ge et al. 2002a; Chakraborty et al. 2002). 3. Development of a Silicon Anamorphic Immersion Grating We plan to apply new grating techniques for fabricating a first silicon anamorphic immersion grating late in Figure 8 shows a hyperfine grating mask with 13 l/mm and mm 2 grating pattern. A chemically mechanically polished silicon disk 3 cm thick and 4 inch in diameter is ready for fabricating. Figure 8 also shows a prototype has been made on a 1 cm thick silicon substrate in A mm 2 was cut and polished for a silicon grism that was used in the first light observations with the IRCAL camera. This grating has produced good IR spectra shown in Figure 7 and provided ~ 10% integrated scattered light at µm and 45% peak grating efficiency. Our new processes should improve the grating surface quality and efficiency of the anamorphic gratings. We have also developed special tools at Penn State Nanofab for handling silicon disks with diameter up to 6 inch and thickness up to 2 inch (Ge et al., 2002b). These tools should provide an immediate step for making high resolution silicon anamorphic immersion gratings. In the future, we will develop new tools for handling silicon disks with diameter up to 12 inch and thickness up to 6 inch for fabricating large silicon anamorphic gratings for Hyperfine Grating Mask (13 l/mm, 80x40 mm 2 ) CMP polished Silicon Disk ((3 cm thick, 4 inch in diameter) very high spectral resolution spectroscopy. 4 inch Figure 8. (left) mm 2 grating mask with 13 l/mm grooves made by the Hyperfine Inc. (middle) a CMP polished silicon disk ready for making an anamorphic grating. (right) An etched grating on a silicon substrate with 100 mm diameter and 10 mm thickness. The image of the camera is reflected off the aligned grating groove steps. A penny is shown next to the grating. 4. Proposed All Sky Surveys for Protoplanets with Silicon Immersion Gratings Once the silicon immersion grating is ready for scientific observations, we plan to conduct an initial survey for protoplanet formation with the ARIES and silicon immersion gratings at the MMT 6.5m telescope in collaboration with Drs. D. McCarthy and R. Angel at Steward Observatory (McCarthy et al. 1998; Sarlot et al. 1999). Due to the use of the adaptive secondary in the MMT AO system, the stellar beams only pass through three warm surfaces (primary, secondary and ARIES window) before they enter

7 the ARIES. The thermal background is therefore extremely low in the thermal IR (e.g. Lloyd-Hart et al. 2000), and this is ideal for detecting weak emission lines from planet formation. The MMT 6.5m + ARIES + a silicon immersion grating will be able to detect the formation of planets over a mass range similar to that probed by current radial velocity surveys (~ M J ). For example, in order to detect the formation of a 1 M J planet at an orbital distance of 1 AU, we must be able to detect the emission produced by an optically thin gap with a predicted width of ~ 0.2 AU (Carr & Najita 1998). With ARIES and a silicon immersion grating, we will be able to study the 4.6 µm CO fundamental lines which are sensitive to residual gas column densities as small as g cm -2. At this column density or higher (10-6 of the minimum solar nebula), the low-j CO lines will be optically thick while the dust continuum will be optically thin, and the line will appear in emission. For typical T Tauri stars (M = 8 mag. at Taurus), this would mean a fairly weak line (~ few percent of the continuum), but one that could be detected with ARIES at high S/N (SNR ~ 40 for a ~ 1 hr exposure) with the AO system (Najita 1999 private communication). We plan to conduct a survey for CO emission lines among all ~ 300 T Tauri stars to the distance of Taurus (d = 140 pc). This will form a sufficient statistical sample for comparison with results from precision velocity surveys. Since we will be sensitive to the same orbital distances probed by RV searches, we can use the results from these surveys to estimate the number of forming planets. If forming planets are detected at the same rate as the detection rate for extra-solar planetary companions (~ 10%, Marcy et al. 2002), we will discover ~ 30 forming planets. If orbital migration and dynamical scattering play a significant role in planet formation and evolution, many more systems will be discovered. Once the silicon immersion gratings are fully tested at the MMT, silicon immersion gratings will be applied to other large ground-based telescopes such as the Keck, Gemini and Magellan etc. for investigating more young planetary systems. ACKNOWLEDGEMENTS The authors would like to thank Joan Najita and Douglas Lin for many stimulating discussions on protoplanet formation and evolution. We thank Paul Kuzmenko and Dino Ciarlo for many stimulating discussions and help about silicon grating fabrication and testing. We thank Jerry Friedman for designing the mechanical mounts for the silicon grisms and staffs at Penn State Physics dept. machine shop for making the grism mounts. We also thank Stephen Fonash for his advice and assistance at Penn State Nanofab. This work was supported by NASA grant NGA , NAG , NAG , NSF AST and the Penn State Eberly College of Science. REFERENCES Carr, J.S., & Najita, J.R., 1998, Studying the Origins of Planetary Systems with NGST, in Science With The NGST (Next Generation of Space Telescope), edited by E.P. Smith and A. Koratkar, ASP Conference Series vol. 133, 163 Carr, J.S., Mathieu, R.D., & Najita, J.R., 2001, Evidence for Residual Material in Accretion Disk Gaps: CO Fundamental Emission from the T Tauri Spectroscopic Binary DQ Tauri, ApJ, 551, 454 Chakraborty, A., Ge, J., Llyod, J., & Graham, J. 2002, Birth of a Be Star, BD : New Spectrscopic Observations Using Adaptive Optics and Silicon Grism, ApJ Letters, to be submitted

8 Dekker, H., An immersion grating for an astronomical spectrograph, in Instrumentation for groundbased optical astronomy, ed. L.B. Robinson (Springer Verlag, 1987), 183 Ge, J., et al., 1999a, Etched Silicon Gratings for NGST, in Proc. Next Generation Space Telescope Science and Technology, ASP Conference Series, Edited by Eric Smith and Knox Long, Vol. 207, 457 Ge, J., et al., 1999b, The First Light of the World's First Silicon Grisms, BAAS, 195, 1504 Ge, J., et al. 2000, High spectral and spatial resolution spectroscopy of YSOs with a silicon grism and adaptive optics, BAAS, 197, 5201 Ge, J. et al. 2001, Development of Silicon Grisms for High-resolution IR spectroscopy, Proc. SPIE, 4485, 393 Ge, J., 2002, Fixed-delay Interferometry for Doppler Extra-solar Planet Detection, ApJ, 571, L165 Ge, J., Kuzmenko, P., Ciarlo, D., & Chakraborty, A., 2002a, First Astronomical IR Spectroscopy with an Etched Silicon Echelle Grism, ApJ Letters, to be submitted Ge, J., et al. 2002b, Breakthroughs in Silicon Grism and Immersion Grating Technology at Penn State, Proc. SPIE, 4841, in press Graf, U.U. et al. 1994, Fabrication and evaluation of an etched infrared diffraction grating, Appl. Opt., 33, 96 Hulthen, E., & Neuhaus, H., 1954, Diffraction gratings in immersion, Nature, 173 Jaffe, D.T., Keller, L.D., & Ershov, O.A., 1998, Micromachined silicon diffraction gratings for infrared spectroscopy, Proc. SPIE, 3354, 201 Kuzmenko, P.J., Ciarlo, D.R., & Stevens, C.G., 1994, Fabrication and testing of a silicon immersion grating for infrared spectroscopy, Proc. SPIE, 2266, 566 Kuzmenko, P.J., & Ciarlo, D.R., 1998, Improving the optical performance of etched silicon gratings, Proc. SPIE, 3354, 357 Lin, D.N.C., Bryden, G., & Ida, S. 1999, Astrophysical Discs - An EC Summer School, Edited by J. A. Sellwood and Jeremy Goodman, Proc. ASP Conf. 160, 207 Lloyd-Hart, M., et al. 2000, Adaptive optics for the 6.5-m MMT, Proc. SPIE, 4007, 167 Marcy, G.W., Cochran, W.D., & Mayor, M., 2000, Extrasolar Planets around Main-Sequence Stars, in Protostars and Planets IV (Book - Tucson: University of Arizona Press; eds Mannings, V., Boss, A.P., Russell, S. S.), p McCarthy, D.W., Burge, J.H., Angel, J.R.P., Ge, J., Sarlot, R.J., Fitz-Patrick, B.C., & Hinz, J.L., 1998, ARIES: Arizona infrared imager and echelle spectrograph, Proc. SPIE, 3354, 750 Sarlot, R.J., McCarthy, D.W., Burge, J.H., & Ge, J., 1999, Optical design of ARIES: the new nearinfrared science instrument for the adaptive f/15 Multiple Mirror Telescope, Proc. SPIE, 3779, 274 Tsang, W.T., & Wang, S., 1975, Preferentially etched diffraction gratings in silicon, J. Appl. Phys., 46, 2163 Wiedemann, G., & Jennings, D.E., 1993, Immersion grating for infrared astronomy, Appl. Opt., 32, 1176