分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: As the number of confirmed exoplanets continues to grow, there is an increased push to spectrally characterize them to determine their atmospheric composition, formation paths, rotation rates, and habitability. However, there is a large population of known exoplanets that either do not transit their star or have been detected via the radial velocity (RV) method at very small angular separations such that they are inaccessible to traditional coronagraph systems. Vortex Fiber Nulling (VFN) is a new single-aperture interferometric technique that uses the entire telescope pupil to bridge the gap between traditional coronagraphy and RV or Transit methods by enabling the direct observation and spectral characterization of targets at and within the diffraction limit. By combining a vortex mask with a single mode fiber, the on-axis starlight is rejected while the off-axis planet light is coupled and efficiently routed to a radiometer or spectrograph for analysis. We have demonstrated VFN in the lab monochromatically in the past. In this paper we present a polychromatic validation of VFN with nulls of $<10^{-4}$ across 15% bandwidth light. We also provide an update on deployment plans and predicted yield estimates for the VFN mode of the Keck Planet Imager and Characterizer (KPIC) instrument. Using PSISIM, a simulation package developed in cooperation with several groups, we assess KPIC VFN's ability to detect and characterize different types of targets including planet candidates around promising young-moving-group stars as well as known exoplanets detected via the RV method. The KPIC VFN on-sky demonstration will pave the road to deployment on future instruments such as Keck-HISPEC and TMT-MODHIS where it could provide high-resolution spectra of sub-Jupiter mass planets down to 5 milliarcseconds from their star.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The formation and evolution pathway for the directly-imaged multi-planetary system HR 8799 remains mysterious. Accurate constraints on the chemical composition of the planetary atmosphere(s) are key to solving the mystery. We perform a detailed atmospheric retrieval on HR 8799~c to infer the chemical abundances and abundance ratios using a combination of photometric data along with low- and high-resolution spectroscopic data (R$\sim$20-35,000). We specifically retrieve [C/H], [O/H], and C/O and find them to be 0.55$^{+0.36}_{-0.39}$, 0.47$^{+0.31}_{-0.32}$, and 0.67$^{+0.12}_{-0.15}$ at 68\% confidence. The super-stellar C and O abundances, yet a stellar C/O ratio, reveal a potential formation pathway for HR 8799~c. Planet c, and likely the other gas giant planets in the system, formed early on (likely within $\sim$1 Myr), followed by further atmospheric enrichment in C and O through the accretion of solids beyond the CO iceline. The enrichment either preceded or took place during the early phase of the inward migration to the planet current locations.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: A benchmark brown dwarf (BD) is a BD whose properties (e.g., mass and chemical composition) are precisely and independently measured. Benchmark BDs are valuable in testing theoretical evolutionary tracks, spectral synthesis, and atmospheric retrievals for sub-stellar objects. Here, we report results of atmospheric retrieval on a synthetic spectrum and a benchmark BD -- HR 7672~B -- with \petit. First, we test the retrieval framework on a synthetic PHOENIX BT-Settl spectrum with a solar composition. We show that the retrieved C and O abundances are consistent with solar values, but the retrieved C/O is overestimated by 0.13-0.18, which is $\sim$4 times higher than the formal error bar. Second, we perform retrieval on HR 7672~B using high spectral resolution data (R=35,000) from the Keck Planet Imager and Characterizer (KPIC) and near infrared photometry. We retrieve [C/H], [O/H], and C/O to be $-0.24\pm0.05$, $-0.19\pm0.04$, and $0.52\pm0.02$. These values are consistent with those of HR 7672~A within 1.5-$\sigma$. As such, HR 7672~B is among only a few benchmark BDs (along with Gl 570~D and HD 3651~B) that have been demonstrated to have consistent elemental abundances with their primary stars. Our work provides a practical procedure of testing and performing atmospheric retrieval, and sheds light on potential systematics of future retrievals using high- and low-resolution data.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The Keck Planet Imager and Characterizer (KPIC) is a series of upgrades for the Keck II Adaptive Optics (AO) system and the NIRSPEC spectrograph to enable diffraction-limited, high-resolution ($R>30,000$) spectroscopy of exoplanets and low-mass companions in the K and L bands. Phase I consisted of single-mode fiber injection/extraction units (FIU/FEU) used in conjunction with an H-band pyramid wavefront sensor. Phase II, deployed and commissioned in 2022, adds a 1000-actuator deformable mirror, beam-shaping optics, a vortex coronagraph, and other upgrades to the FIU/FEU. The use of single-mode fibers provides a gain in stellar rejection, a substantial reduction in sky background, and an extremely stable line-spread function on the spectrograph. In this paper we present the results of extensive system-level laboratory testing and characterization showing the instrument's Phase II throughput, stability, repeatability, and other key performance metrics prior to delivery and during installation at Keck. We also demonstrate the capabilities of the various observing modes enabled by the new system modules using internal test light sources. Finally, we show preliminary results of on-sky tests performed in the first few months of Phase II commissioning along with the next steps for the instrument. Once commissioning of Phase II is complete, KPIC will continue to characterize exoplanets at an unprecedented spectral resolution, thereby growing its already successful track record of 23 detected exoplanets and brown dwarfs from Phase I. Using the new vortex fiber nulling (VFN) mode, Phase II will also be able to search for exoplanets at small angular separations less than 45 milliarcseconds which conventional coronagraphs cannot reach.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Coronagraphs allow for faint off-axis exoplanets to be observed, but are limited to angular separations greater than a few beam widths. Accessing closer-in separations would greatly increase the expected number of detectable planets, which scales inversely with the inner working angle. The Vortex Fiber Nuller (VFN) is an instrument concept designed to characterize exoplanets within a single beam-width. It requires few optical elements and is compatible with many coronagraph designs as a complementary characterization tool. However, the peak throughput for planet light is limited to about 20%, and the measurement places poor constraints on the planet location and flux ratio. We propose to augment the VFN design by replacing its single-mode fiber with a six-port mode-selective photonic lantern, retaining the original functionality while providing several additional ports that reject starlight but couple planet light. We show that the photonic lantern can also be used as a nuller without a vortex. We present monochromatic simulations characterizing the response of the Photonic Lantern Nuller (PLN) to astrophysical signals and wavefront errors, and show that combining exoplanet flux from the nulled ports significantly increases the overall throughput of the instrument. We show using synthetically generated data that the PLN detects exoplanets more effectively than the VFN. Furthermore, with the PLN, the exoplanet can be partially localized, and its flux ratio constrained. The PLN has the potential to be a powerful characterization tool complementary to traditional coronagraphs in future high-contrast instruments.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Directly imaging Earth-like exoplanets (``exoEarths'') with a coronagraph instrument on a space telescope requires a stable wavefront with optical path differences limited to tens of picometers RMS during exposure times of a few hours. While the structural dynamics of a segmented mirror can be directly stabilized with telescope metrology, another possibility is to use a closed-loop wavefront sensing and control system in the coronagraph instrument that operates during the science exposures to actively correct the wavefront and relax the constraints on the stability of the telescope. In this paper, we present simulations of the temporal filtering provided using the example of LUVOIR-A, a 15~m segmented telescope concept. Assuming steady-state aberrations based on a finite element model of the telescope structure, we (1)~optimize the system to minimize the wavefront residuals, (2)~ use an end-to-end numerical propagation model to estimate the residual starlight intensity at the science detector, and (3)~predict the number of exoEarth candidates detected during the mission. We show that telescope dynamic errors of 100~pm~RMS can be reduced down to 30~pm~RMS with a magnitude 0 star, improving the contrast performance by a factor of 15. In scenarios where vibration frequencies are too fast for a system that uses natural guide stars, laser sources can increase the flux at the wavefront sensor to increase the servo-loop frequency and mitigate the high temporal frequency wavefront errors. For example, an external laser with an effective magnitude of -4 allows the wavefront from a telescope with 100~pm~RMS dynamic errors and strong vibrations as fast as 16~Hz to be stabilized with residual errors of 10~pm~RMS thereby increasing the number of detected planets by at least a factor of 4.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Gaps in circumstellar disks can signal the existence of planetary perturbers, making such systems preferred targets for direct imaging observations of exoplanets. Being one of the brightest and closest stars to the Sun, the photometric standard star Vega hosts a two-belt debris disk structure. Together with the fact that its planetary system is being viewed nearly face-on, Vega has been one of the prime targets for planet imaging efforts. Using the vector vortex coronagraph on Keck/NIRC2 in Ms-band at 4.67 $\mu$m, we report the planet detection limits from 1 au to 22 au for Vega with an on-target time of 1.8 h. We reach a 3 Jupiter mass limit exterior to 12 au, which is nearly an order of magnitude deeper than existing studies. Combining with existing radial velocity studies, we can confidently rule out the existence of companions more than ~8 Jupiter mass from 22 au down to 0.1 au for Vega. Interior and exterior to ~4 au, this combined approach reaches planet detection limits down to ~2-3 Jupiter mass using radial velocity and direct imaging, respectively. By reaching multi-Jupiter mass detection limits, our results are expected to be complemented by the planet imaging of Vega in the upcoming observations using the James Webb Space Telescope to obtain a more holistic understanding of the planetary system configuration around Vega.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The detection of satellites around extrasolar planets, so called exomoons, remains a largely unexplored territory. In this work, we study the potential of detecting these elusive objects from radial velocity monitoring of self-luminous directly imaged planets. This technique is now possible thanks to the development of dedicated instruments combining the power of high-resolution spectroscopy and high-contrast imaging. First, we demonstrate a sensitivity to satellites with a mass ratio of 1-4% at separations similar to the Galilean moons from observations of a brown-dwarf companion (HR 7672 B; Kmag=13; 0.7" separation) with the Keck Planet Imager and Characterizer (KPIC; R~35,000 in K band) at the W. M. Keck Observatory. Current instrumentation is therefore already sensitive to large unresolved satellites that could be forming from gravitational instability akin to binary star formation. Using end-to-end simulations, we then estimate that future instruments such as MODHIS, planned for the Thirty Meter Telescope, should be sensitive to satellites with mass ratios of ~1e-4. Such small moons would likely form in a circumplanetary disk similar to the Jovian satellites in the solar system. Looking for the Rossiter-McLaughlin effect could also be an interesting pathway to detecting the smallest moons on short orbital periods. Future exomoon discoveries will allow precise mass measurements of the substellar companions that they orbit and provide key insight into the formation of exoplanets. They would also help constrain the population of habitable Earth-sized moons orbiting gas giants in the habitable zone of their stars.