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Observation Projects |
Instrumentation Projects | Theoretical Projects |
We plan to launch a search for extra-solar planets around nearby stars using a new version of the inteferometric instrument in the optical at the KPNO 2.1m starting in August 2003. This new version will have much higher throughput than the prototype. For example, the total efficiency from the sky to the detector (excluding iodine absorption) is ~20% for the new survey instrument vs. ~ 5% for the prototype. This will allow us to reach about 20 m/s Doppler precision for V = 12 stars, much deeper than most of current planet surveys with echelle spectrographs. Our next step is to apply this instrument for multi-object object observations. Coupling this instrument with a wide field telescope such as the WIYN 3.5m telescope will possibly revolutionize Doppler radial velocity survey field. The radial velocity data obtained from this all sky survey will not only detect thousands of extrasolar planets, but also provide important informations about physical and chemical properties of millions of nearby stars such as stellar activity, metallicity. Back to Top
Postdoc, A. Chakraborty, students, J. Debes and J. Wang, and Prof. Ge conducted a high angular resolution (~ 0.2 arcsec) search for faint companions using our PIRIS with a natural guide star adaptive optics system at NIR wavelengths at the Mt. Wilson 100-inch in 2001 and 2002. We observed G and K type stars in the solar neighborhood, as well as in the nearby young star clusters like MBM12 and MBM20. We made the discovery of two faint companions to main sequence stars: HD190067 and HIP13855. Based on ages and distances to these two primary stars, we were able to estimate masses of these faint companions (~ 0.08 Msun for HD 190067B, and ~ 0.15 Msun for HIP 13855B). A comparison between the measured properties (absolute magnitu de and age) of these two companions and theoretical evolution model curves from Baraffe et al. (1998 and 2001) incidates both of the faint sources are extreme low mass stars close to the hydrogen burning limit. If HD190067B is less than 1 Gyr then it could be a transition object.
A survey of ~ 20 nearby solar type stars in 2002 has identified a brown dwarf companion candidate around a G type star. This companion has 65-70 Jupiter mass. Its spectral type is between L6-L8 based on their IR colors. More results will be reported later. Back to Top
Our current effort in observational cosmology is to test the Big Bang theory by measuring Cosmic Microwave Background Radiation (CMBR) temperatures at high redshifts through observing neutral carbon (C I) fine-structure transitions in quasar absorption line spectra. The existence of the CMBR is regarded as the best evidence for a primeval expanding of the Universe, the Big Bang. The present-day temperature of the CMBR is accurately measured by the COBE FIRAS instrument to be T(0) = 2.728 K. The COBE experiment also provides data with remarkable sensitivity on the isotropy and blackbody nature of the spectrum. However, this result by itself does not constitute a proof of the cosmological nature of the CMBR. If the universe really started from a homogeneous and isotropic collapsed state, the simplest prediction of such a model is that the temperature of this blackbody radiation has always been spatially uniform and increases linearly with redshift, i.e., T(z) = 2.728 (1+z) K. Therefore, the measurements of CMBR temperatures at high redshifts provide direct and important evidence for the Big Bang model. As is widely known, the best experimental method available to measure T(z) is provided by absorption lines from molecules, atoms and ions. The small energy separations in atomic fine structures of C I, C II, O I, N II, Si II etc. are well suited for these measurements. Among all available species, C I is the best probe since it has the smallest energy separation in its fine-structure levels, corresponding to 23.6 K for the J=0-1 separation. The high redshift CMBR would excite C I more efficiently than the present-day CMBR due to the predicted higher temperature of background radiation. Therefore, measurements of C I excitation can yield sensitive upper limits on the temperature of the CMBR in the past. Until now, considerable progress has been made by several groups in measuring CMBR temperatures at high redshifts using C I and C II as probes. We have detected weak C I absorption lines from two high z Damped Lyman alpha quasar absorbers (DLAs), the z = 1.97 DLA toward Q 0013-004 and z = 2.34 DLA toward Q 1232+815, with the MMT Blue and Red Channel spectrographs (with Jill Bechtold, John Black, Vasha Kulkarni). The CMBR temperature at z = 1.97 is measured to be about 7.9 K after local excitations to the C I fine structure having been removed. This temperature is consistent with the Big Bang cosmology prediction. We have also measured the upper limit of the CMBR temperature at z = 2.34. The upper limit of 15.7 K is also consistent with the Big Bang prediction. However, in order to obtain the real value of the CMBR temperature at these systems, we need to measure the local contributions to the excitation of the C I fine-structure, such as collision and UV pumping. We found these contributions are quite significant in the local diffuse clouds. The following figure summarizes all the measurements, which are consistent with the theoretical prediction.
We have detected very strong molecular hydrogen absorption bands in the z = 1.97 and z = 2.34 damped systems, the strongest case of H2 absorption in the early universe. The physical condition in this system is very similar to that of local diffuse clouds, such as density and kinetic temperature. However, the UV radiation field strength in both systems are a factor of ten higher than the local value, indicating that either these two systems are close to star formation regions or the global star formation rates are much higher than the Milky Way's rate.
Together with other measurements of H2 upper limits from our survey and literatures, we find a strong correlation between the fractional H2 abundance and the dust depletion. Therefore, H2 must efficiently form in these high redshift galaxies through dust-catalytic processes.
We (Profs. Jill Bechtold, Dave Meyer and I) have been awarded a total of 26 orbits of the HST STIS time to obtain high S/N and high resolution STIS spectra (R ~ 20,000) of Q 1331+170. The UV spectra will be used for observing H2 absorption in a z = 1.77 DLA toward this quasar. Comparing to previous measurements of C I excitation temperature by the Keck HIRES, we plan to use the measurements of the physical conditions through the H2 absorption lines to put a strong constraint on the upper limit of the CMB radiation temperature at this redshift to test validity of the Big Bang cosmology.
We plan to launch a long term HET project for searching for the H2 and C I in DALs at z ~ 3 using the High Resolution Spectrograph and Medium Resolution Spectrograph once it is ready for observing faint objects. The picture below shows a simulated HRS spectrum at a spectral resolution of R ~ 30,000 and a S/N ~ 10. It is a mixed spectrum of H2 and numerous Lyman alpha clouds. This kind of data can provide high sensitivity for detecting H2 at z ~ 2.8-3.6, e.g., a non-detection 3 sigma upper limit is N(H2) ~ 1x1015 cm2, which is well below that shown in our previous detected DLAs such as Q 0013 and Q 1232. Therefore, the new sample can be combined with our previous sample obtained at the old MMT, and the HST archive to study possible evolution of H2 formation at different redshifts
We also plan to study metal abundances at z > 4 with JCam coupled with the MRS at the HET. The MMT 6.5m with PISCES wide field camera, ARIES plus silicon grisms developed by Jian Ge's research group at Penn State will be used for near-IR imaging and spectroscopy of candidate galaxies associated with DLAs. Gemini FLAMINGOS with our silicon grisms will also be used for similar studies. Back to Top
Venus Transit Observations Back
The upcoming Venus transits across the solar disk
are rare events, occurring in pairs less than once per century. Prof. Ge's
group (Dr. D. Ren, Ms. C.Mendelowitz and Prof. Ge) and his collaborators,
Dr. Sara Seager and her group at the Carnegie
Institute of Washington, are working on preparing a 3-D imaging spectrograph
for capturing transit spectra. This data will be used for testing the feasibility
of detection of terrestrial planet transmission spectra within the signal
of a non-spatially resolved star for future space missions. The eventual
detection and characterization of extra-solar terrestrial planets, especially
those with signs of atmospheric modification by life, will have a huge
impact on science and society.
High Redshift Gamma Ray Burst Afterglows Back
We (Ge, Zhang, Abel, Meszaros, Schneider and graduate students) are preparing the Penn State near IR Imager and Spectrograph (PIRIS) for performing ground-based follow-up observations of the Gamma-ray burst (GRB) sources detected by the Swift GRB mission in 2003-2006. The high sensitivity of PIRIS and quick instrument response (within 10 min) will enable the capture of the brightest phase of the GRB afterglows, to obtain high-resolution spectrum (R ~ 200) of the afterglow, and to measure their redshifts. The data will permit a study of spectral features associated with GRB hosts or intergalactic medium, probe the prompt emission phase, and investigate bridges between the prompt emission phase and afterglow phase. Unlike most of the ground-based follow-up observations that are limited to optical wavelengths, our observations will be conducted in the near-IR (0.8-2.4 microns). GRBs at redshifts between z=5.6-18.8 will be identified through detecting the unique strong Lyman alpha absorption trough due to the Gunn-Peterson effect. Detection of these high redshift objects will provide one of the most powerful tools for investigating the first generation of star formation and the reionization era in the early universe. IR observations can also help to identify many GRBs deeply embedded in the dusty environment and reveal the nature of the progenitors of GRBs at moderate redshifts.
PIRIS coupled with the Mt. Wilson 100-inch telescope has the ability and the unique advantage to fulfill this task, long before the use of the Next Generation Space Telescope for such an exciting study in the next decade. Assuming that the GRBs at z = 5.6-18.8 have the same afterglow luminosity as those at z = 0.3 -4.5, PIRIS can detect all of them at a spectral resolving power of R ~ 200 within ~1-2 hours after the bursts. At lower spectral resolution such as R ~ 20, we can monitor GRBs for ~ 10 hours as they fade. During the ground-based GRB follow-up observations, we will also provide sub arsec precision coordinate information of the GRBs to GRB Coordinates Network (GCN), allowing quick follow-up afterglow observations by other groups all over the world.
High Spectral and Spatial Resolution Near-IR Spectroscopy of Young Stars and their close Companions Back
Several T Tauri and Ae/Be stars and their close companions were observed by Prof. Ge, J. Lloyd and D. Gavel with a high dispering silicon grism in the near-IR IRCAL camera at the Lick 3m telescope with adaptive optics in September 2000. The silicon grism, developed by Prof. Ge and his former LLNL development team, provides R = 2,000 in the K band with a 5 mm diameter cold pupil. With a CaF2 grism cross-disperser, the entire K band spectrum is covered by the 256x256 HgCdTe PICNIC array. This was achieved by the silicon echelle grism with 13.3 lines/mm, nearly a factor of two times coarser than any commercially available echelles. The new grating technique, based on photolithography and anisotropic chemical etching, demonstrates its flexibility for making any period of grating grooves to produce an echellogram match with any detector dimension.
Initial spectroscopy results of BD +65 1638 show a very broad Br gamma absorption feature with a Full Width at Half Maximum (FWHM) of 550(+/-50) km/s. This observation provides the first evidence that this star may be a progenitor of a fast rotating Be star. Our high spatial IR spectroscopy of T Tauri N and S companions shows both companions have similar Br gamma emission lines. The flux levels match recent results from the Keck, indicating the great potential of silicon echelle grisms for high resolution spectroscopy at moderate size cameras and telescopes. More obseravtions of young stellar objects and their close companions will be observed at the Lick 3m, Mt. Wilson 100-inch and other telescopes.
Studies of Central Nuclear Regions of nearby Seyfert Galaxies with Adaptive Optics Back
We (Profs. Ge, and C. Max (UCSC & LLNL)) have obtained adaptive optics high spatial resolution K-band spectra, narrow and broad band images of NGC 6240 at the Lick 3m telescope.
Data analysis by student, Ms. T. Bogdanovic, shows evidence of the presence of circumnuclear shock and young and old starburst populations around the nuclear region. The narrow band H2 image shows stronly perturbed morphology of the molecular gas in the nuclear region.
Instrumentation Projects
Silicon Immersion Grating Optics Technology for IR Spectroscopy Back
Silicon immersion diffraction gratings fabricated by photolithography and anisotropic chemical etching techniques being developed by an interdisciplinary team led by Jian Ge will be key dispersing elements for next generation space and ground-based IR spectroscopic instruments. Due to its very high refractive index (n = 3.4 at 2.2 micron), a silicon immersion grating can provide more than three times dispersion of a conventional reflective grating of equal length. Therefore, silicon immersion gratings will enable very compact IR spectroscopic instruments while provide high dispersion power.
We have successfully developed the world's first silicon grisms in 1999. They have 10x10 mm2 etched grating area and 46 deg wedge angles as shown in the following figure. The key team members include Jian Ge, and senior engineers, Dino Ciarlo and Paul Kuzmenko. The first light of one of the grisms at the Lick 3m telescope with the IRCAL near-IR camera and adaptive optics has demonstrated a diffraction-limited spectral resolution, R = 5,000 at 2.2 micron with a pupil diameter of only 5 mm. This spectral resolution is the highest ever obtained with grisms. The measured total grating efficiency is 36% and the integrated scattered light level from the grism is about 30%. Coupled with the Lick AO system, it allows efficient IR spectroscopy at very high spatial resolution at 0.2 arcsec. Combined with a conventionally made CaF2 grism cross-disperser, it allows a complete wavelength coverage in the K band.
In 1999, we (Prof. Ge, D. Ciarlo and P. Kuzmenko (LLNL)) developed a another technique which reduces grating surface roughness from the original rms 50 nm to 20 nm, which has reduced the total integrated scattered level to about 8% in the K band. A new set of the grisms has been used for scientific observations of T Tauri stars and Ae/Be stars and their companions at the Lick 3m in September 2000. The total grating efficiency has been increased to 45%.
The silicon immersion gratings promise a major impact in IR spectroscopy. The silicon grisms promise a very convenient and inexpensive way to implement intermediate and high spectral resolution in any existing IR camera. The silicon immersion gratings offer high efficiency and very high spectral resolving power (R > 100,000) in the IR for the first time.
Prof. Ge's Penn State group (Prof. Ge, S. Miller, D. McDavitt, J. Bernecker, A. Chakraborty, J. Wang and J. Friedman) has developed new etching processes based on TMAH instead of previous KOH for fabricating silicon grisms and immersion gratings, taking advantage of state-of-the-art nanofabrication facility, Nanofab, at Penn State. The rms grating surface roughness has been reduced to ~ 3 nm and the intergrated scattered light level is less than 1%. This new technique has been used for fabricating new generations of silicon grisms and also silicon immersion gratings with up to 4 inch etched grating size. These grisms will be used in the NGST prototype near-IR multi-object spectrograph led by Dr. Harvey Moseley at GSFC, the FLAMINGOS near-IR MOS led by Dr. Richard Elston at University of Florida and PISCES near-IR wide field camera and Arizona Imager and Echelle Spectrograph (ARIES) led by Dr. Don McCarthy at Steward Observatory.
Several silicon immersion gratings are being developed at Penn State and used in the Arizona IR Imager and Echelle Spectrograph (ARIES) at the MMT 6.5m. A silicon immersion grating with 2 inch pupil diameter will provide R = 120,000 in 1.2-5.5 micron. The main science goal for such high resolution spectroscopy is to detect emission lines of CO fundamental band at 4.6 micron caused by the residual gas in the dynamic gaps caused by the young planets. The high resolution spectroscopy allows us to study the location, total mass of the planets, density and temperature of the residual gas with the planet formation. The very low thermal emissivity of the MMT 6.5m adaptive optics provide great sensitivity for this exciting study. Back to Top
A Dispersed Fixed-delay Interfereometer for Doppler Extra-solar Planet Searches
BackA prototype dispersed fixed-delay interferometer has been developed at Penn State by Prof. Ge's team, including J. van Eyken, S. Mahadevan, C. DeWitt, J. Liu, Prof. J. Ge and Dr. S. Shaklan (JPL), collaborated with Mike Rushford at LLNL, and has been used at the HET 9m and Palomar 5 m telescopes in 2001. It was used again at the KPNO 2.1m telescope in 2002 and routinely provided observations of stars as faint as V = 7.6. The total instrument throughput from the sky to the detector is ~5% (excluding iodine absorption), comparable to or slightly higher than that for current echelle instruments. A new f/3 instrument is being developed at Penn State and will provide ~ 20% total throughput by using both interferometer outputs, volumn phase holographic grating and better designed optics. It will see first light at the KPNO 2.1m telescope in the summer 2003 and a long term survey for extrasolar planet will be launched shortly after the instrument commissioning.
Students working in this project have been exposed to almost every aspects of the instrument development, from optical-mechanical design, optical system alignment, system integration, CCD camera system testing to data taking and analysis. Back to Top
Coronagraph Techniques
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Prof. Ge's team (J. Debes, D. Ren, A. Watson, C. Mendelowitz) and his collaborators, at Princeton University, Harvard-Smithsonian Center for Astrophysics and Ball Aerospace Technology Inc. are developing new coronagraph techniques for the NASA Terrestrial Planet Finder mission, to be launched in 2015. One approach is to use a special shaped pupil such as a gaussian shaped one for reaching very deep contrast in some of the image areas as shown in the Figure.
They have successfully demonstrated the feasibility of a shaped pupil coronagraph for high contrast imaging in the lab, as well as, at the astronomical telescope. In the lab, they have reached ~ 10-6 contrast at ~ 5 times diffraction-limited Airy disk size. At the Mt. Wilson 100inch telescope with a high order adaptive optics system, they have reached ~ 10-4 at the ~ 10 times diffraction-limited Airy disk size. They have successfully detected faint companions around nearby stars using this new high contrast imaging technique. They are working on developing new techniques for further improvement in image contrast. Their goal is to reach ~ 10-10 contrast to allow TPF to detect Earth-like planets in space in next decade. Prof. Ge's group is also taking advantage of the new high contrast imaging techniques to search for brown dwarfs and giant planets around nearby stars. To date, they have detected two transition objects, and a brown dwarf candidates.
Integral Field Optics and CUBE Machine
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Currently we (D. Ren, J. Friedman and Prof. Ge) are constructing a prototype integral field unit (IFU) optical system by using advanced image slicer technique for 3-D imaging spectroscopy. It is designed for future NASA missions to the Mars and other planetary objects in collaborations with Profs. J. Lunine, R. Brown and R. Yelle at Univ. of Arizona and L. Soderblom at U.S. Geological Survey. The reflective design of the IFU allows it operate in cryogenic environment and broad band. A combination of this advanced image slicer IFU with silcionb grisms we are developing at Penn State enables a very compact, light weight, high efficiency and high spectral resolution 3-D instrument design for space missions.
We plan to develop IFU based on fiber bundles for optical 3-D imaging spectroscopy. A single mode fiber bundle can help to reduce scattered light in the optical system and can be used in the TPF system.
Penn State near IR Imager and Spectrograph (PIRIS)
BackA near-IR imaging spectrograph has been constructed for testing new instrument components we developed at Penn State in the lab and also observing at observatories.The new instrument components include silicon grisms, silicon immersion gratings, shaped pupil masks, band-limited masks and advanced image slicer integral field optics. It is designed for diffraction-limited operations in the J, H and K bands with a 256x256 PICNIC IR array. It has an aperture wheel which has 6 open slots to hold slits for spectroscopy. It has a filter wheel before the cold pupil and a grism wheel after the pupil. Each of the wheels can hold 10 filters or grisms. With a 4 mm pupil diameter, grisms including our developed silicon grisms can provide R up to 4000. This instrument has been used at the Mt. Wilson for high spatial resolution imaging surveys for faint companions around nearby stars and also successful demonstration of high contrast imaging with shaped pupil coronagraphs.
Collaborated with a team led by Drs. Harvey Moseley, Bruce Woogate at NASA Goddard Space Flight Center (GSFC) , we are working on developing a prototype near IR multi-object spectrograph for the NGST. The key elements for this instrument are a micro-shutter array and silicon grisms. The microshutter array is being developed at GSFC. The silicon grisms are being developed at Penn State. The optical design has been studied by Prof. Ge, Dr. B. Dean and C. Marx at GSFC. It provides 6 arcmin field of view. The spectral resolutions includes R = 50, 2000, 4500. It covers wavelengths from 0.6 micron to 5.5 micron with an 1kx1k InSb IR array. The construction of the instrument will be in 2003.
The instrument is mainly designed to be used at the APO 3.5m telescope for for scientific observations. The main science we plan to conduct is multi-object spectroscopy of high redshift galaxies, young stellar objects and follow-up observations of Gamma ray bursts at high redshifts. Back to Top
In a dispersed fixed-delay interferometer (FDI), a fixed optical delay is applied to one of the interometer beams. The interference happens at very high interference order. The Doppler RV motion will shift the fringes of stellar absorption lines to neighboring orders. The corresponding Doppler velocity shift is proportional to phase shift of the intereference fringes of stellar absorption lines. Therefore, Doppler radial velocity can be derived from fringe phase shifts. In order to increase fringe visibility for precision measurements of the fringe phase shifts, stellar fringes are dispersed by a moderate resolution post-disperser and recorded by a 2-D detector. This operation is critrical when observing faint star light.
It is very clear that the use of a dispersed fixed-delay interferometer for Doppler RV measurements is completely different from the current echelle approach. Due to the simple and stable instrument responses with the interferometer, the dispersed inteferometer method can provide better Doppler sensivity than the echelle. The independence of Doppler sensitivity from the post-disperser resolving power in the interferometer approach opens up new possibilities for RV studies. The use of low resolution but high efficiency post-dispersers can significantly boost the overall detection efficiency, dramatically reduce the instrument size and cost and allow single dispersion order operations for multiple object observations. Full sky coverage for an RV survey for planets becomes possible with wide field telescopes. Multiple object capability is one of the most significant advantages for this interferometer approach.
The original idea for using a fixed-delay interferometer for high precision Doppler RV measurements was proposed by two groups (Gorskii & Lebedev 1977; Beckers & Brown 1978). This interferometer with a narrow bandpass has been successfully used for very high Doppler precision measurements of the sun (~ 3 m/s, Kozhevatov et al. 1995, 1996; sub m/s precision for the GONG measurements, Harvey 2002 private communication). The concept of combining of a fixed-delay interferometer with a moderate resolution spectrometer, or a post-disperser, for broad band operations for high precision stellar Doppler measurements was proposed by Dave Erskine at LLNL in 1997. The initial lab experiments and telescope observing with a prototype demonstrated its feasibility (Erskine & Ge 2000; Ge et al. 2002a). A theory for this new instrument concept was developed by Jian Ge (Ge 2002).
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