arXiv subjects

Yu Komatsu

Publications and source records attributed to Yu Komatsu.

5 recordsLinked to original sources

arXiv2026

White Paper on Phototrophic Biosignatures: Research Priorities for the Search for Life on Other Worlds

Photosynthesis is of prime interest in the telescopic search for life beyond the Solar System, because, on Earth, oxygenic photosynthesis produces two strong "biosignatures," global scale signs of life that can be seen from space: atmospheric oxygen and the Vegetation Red Edge (VRE). The VRE is the spectral reflectance signature of plant leaves, characterized by a step-like increase in reflectance from the red to the near-infrared. The absorption in the red is due to chlorophyll $\textit{a}$ (Chl $\textit{a}$). While Chl $\textit{a}$ dominates our planet, the Earth harbors diverse phototrophic organisms in niche environments possessing other pigments that produce edge-like spectral features across the UV-VIS-NIR, naturally suggesting diverse signatures that could be found on other planets where phototrophic life is adapted to other stars. However, the astrobiology community is very much at an early stage in its ability to constrain the probability that an observation of another planet has detected a sign of photosynthetic life. This white paper identifies critical research questions to advance to a predictive capability the search for phototrophic biosignatures. These questions pertain to the origins, key features, diversity, and potential for alternative adaptations in fundamental aspects of light harvesting; the electron transfer pathway in photosynthesis; rhodopsin-based proton-pumping; and carbon fixation. We discuss the need to constrain how evolution and ecology affect the scaling up of these molecular mechanisms to be potentially detectable by a direct imaging mission. The research questions and recommendations presented here are cross-linked to those posed by the NASA Astrobiology Strategy 2015, and to the Focus Areas of the upcoming NASA Decadal Astrobiology Exploration Strategy (DARES).

astro-ph.IM↗

arXiv2023

The Automated Reaction-Pathway Search reveals the Energetically Favorable Synthesis of Interstellar CH3OCH3 and HCOOCH3

Recent astronomical observations have shown that interstellar complex organic molecules (COMs) exist even in cold environments ($\sim$10 K), while various interstellar COMs have conventionally been detected in the hot gas ($\gtrsim$ 100 K) in the vicinity of high-mass and low-mass protostars. However, the formation pathway of each interstellar COM remains largely unclear. In this work, we demonstrate that an automated reaction path search based on transition state theory, which does not require predetermined pathways, is helpful for investigating the formation pathways of interstellar COMs in the gas phase. The exhaustive search within electronic ground states helps elucidate the complex chemical formation pathways of COMs at low temperatures. Here we examine the formation pathways of dimethyl ether (CH$_3$OCH$_3$) and methyl formate (HCOOCH$_3$), which are often detected in the cold and hot gas of star-forming regions. We have identified a barrierless and exothermic formation path of CH$_3$OCH$_3$ by reaction between neutral species; CH$_3$O + CH$_3$ $\rightarrow$ H$_2$CO $\cdots$ CH$_4$ $\rightarrow$ CH$_3$OCH$_3$ is the most efficient path in the large chemical network constructed by our automated reaction path search and is comparable with previous studies. For HCOOCH$_3$, we obtain complex pathways initiated from reactions between neutral species; HCOO and CH$_3$ generate HCOOCH$_3$ and its isomers without external energy. However, we also identified the competing reaction branches producing CO$_2$ + CH$_4$ and CH$_3$COOH, which would be more efficient than the formation of HCOOCH$_3$. Then the gas-phase formation of HCOOCH$_3$ through reactions between neutral species would not be efficient compared to the CH3OCH$_3$ formation.

astro-ph.GA↗

arXiv2023

A Possible Converter to Denoise the Images of Exoplanet Candidates through Machine Learning Techniques

The method of direct imaging has detected many exoplanets and made important contribution to the field of planet formation. The standard method employs angular differential imaging (ADI) technique, and more ADI image frames could lead to the results with larger signal-to-noise-ratio (SNR). However, it would need precious observational time from large telescopes, which are always over-subscribed. We thus explore the possibility to generate a converter which can increase the SNR derived from a smaller number of ADI frames. The machine learning technique with two-dimension convolutional neural network (2D-CNN) is tested here. Several 2D-CNN models are trained and their performances of denoising are presented and compared. It is found that our proposed Modified five-layer Wide Inference Network with the Residual learning technique and Batch normalization (MWIN5-RB) can give the best result. We conclude that this MWIN5-RB can be employed as a converter for future observational data.

astro-ph.EP↗

arXiv2023

Photosynthetic Fluorescence from Earth-Like Planets around Sun-Like and Cool Stars

Remote sensing of the Earth has demonstrated that photosynthesis is traceable as the vegetation red edge (VRE), which is the steep rise in the reflection spectrum of vegetation, and as solar-induced fluorescence. This study examined the detectability of biological fluorescence from two types of photosynthetic pigments, chlorophylls (Chls) and bacteriochlorophylls (BChls), on Earth-like planets with oxygen-rich/poor and anoxic atmospheres around the Sun and M dwarfs. Atmospheric absorption, such as H2O, CH4, O2, and O3, and the VRE obscure the fluorescence emissions from Chls and BChls. We found that BChl-based fluorescence for wavelengths of 1000-1100 nm, assuming the spectrum of BChl b-bearing purple bacteria, could provide a suitable biosignature but only in the absence of the water cloud coverage or other strong absorbers near 1000 nm. The Chl fluorescence is weaker for several reasons, e.g., spectral blending with the VRE. The apparent reflectance excess is greatly increased in both Chl and BChl cases around TRAPPIST-1 due to fluorescence and stellar absorption lines. This could be a promising feature for detecting the fluorescence around ultracool red dwarfs by follow-up ground-based observations with high spectral resolution; however, it requires a long time around Sun-like stars, even for a LUVOIR-like space mission. Moreover, the simultaneous detection of fluorescence and VRE is key to identifying traces of photosynthesis because absorption, reflectance, and fluorescence are physically connected. For further validation of fluorescence detection, the nonlinear response of biological fluorescence as a function of light intensity could be considered.

astro-ph.EP↗

arXiv2022

Community Report from the Biosignatures Standards of Evidence Workshop

The search for life beyond the Earth is the overarching goal of the NASA Astrobiology Program, and it underpins the science of missions that explore the environments of Solar System planets and exoplanets. However, the detection of extraterrestrial life, in our Solar System and beyond, is sufficiently challenging that it is likely that multiple measurements and approaches, spanning disciplines and missions, will be needed to make a convincing claim. Life detection will therefore not be an instantaneous process, and it is unlikely to be unambiguous-yet it is a high-stakes scientific achievement that will garner an enormous amount of public interest. Current and upcoming research efforts and missions aimed at detecting past and extant life could be supported by a consensus framework to plan for, assess and discuss life detection claims (c.f. Green et al., 2021). Such a framework could help increase the robustness of biosignature detection and interpretation, and improve communication with the scientific community and the public. In response to this need, and the call to the community to develop a confidence scale for standards of evidence for biosignature detection (Green et al., 2021), a community-organized workshop was held on July 19-22, 2021. The meeting was designed in a fully virtual (flipped) format. Preparatory materials including readings, instructional videos and activities were made available prior to the workshop, allowing the workshop schedule to be fully dedicated to active community discussion and prompted writing sessions. To maximize global interaction, the discussion components of the workshop were held during business hours in three different time zones, Asia/Pacific, European and US, with daily information hand-off between group organizers.

astro-ph.IM↗