arXiv subjects

David Graham

Publications and source records attributed to David Graham.

6 recordsLinked to original sources

arXiv2026

Signatures of photospheric convection throughout the solar atmosphere: the EVE Sun-as-a-star mHz continuum

Convectively driven motions in the solar photosphere can generate broadband Doppler variability across the chromosphere, transition region and corona. Here we investigate this variability using "Sun-as-a-Star" observations from the Extreme Ultraviolet Variability Experiment (EVE) aboard the Solar Dynamics Observatory, constructing high signal-to-noise Doppler power spectra from incoherently summed 3-hour sequences of the centroid wavelengths of emission lines that span wavelengths 35-104 nm. The spectra reveal a broad power-spectral continuum with two Harvey-like components, one of which extends to the Nyquist frequency at 50 mHz with a steep power-law tail. Lines formed in the corona, as compared with those of the chromosphere/transition region, have substantially less Doppler amplitude in the 5 mHz Harvey component associated with granulation-scale convection. Based on the observed continuum, there is no evidence (in any of the 26 lines studied) for Kolmogorov turbulence, which predicts a flat continuum component with Doppler variance ~ f^{-5/3) as a function of frequency f. The total inferred non-thermal RMS velocities (>0.1 mHz) are of order 15 km/s, consistent with previous coronal "microturbulence" estimates from non-thermal line widths. These observations provide the first clear detection of Sun-as-a-star EUV Doppler variability above about 10 mHz and demonstrate the potential of full-disk EUV spectroscopy to probe turbulent energy transport throughout the solar atmosphere.

astro-ph.SR↗

arXiv2025

Olmo 3

We introduce Olmo 3, a family of state-of-the-art, fully-open language models at the 7B and 32B parameter scales. Olmo 3 model construction targets long-context reasoning, function calling, coding, instruction following, general chat, and knowledge recall. This release includes the entire model flow, i.e., the full lifecycle of the family of models, including every stage, checkpoint, data point, and dependency used to build it. Our flagship model, Olmo 3 Think 32B, is the strongest fully-open thinking model released to-date.

cs.CL↗

arXiv2024

History and Habitability of the LP 890-9 Planetary System

We present numerous aspects of the evolution of the LP 890-9 (SPECULOOS-2/TOI-4306) planetary system, focusing on the likelihood that planet c can support life. We find that the host star reaches the main sequence in 1 Gyr and that planet c lies close to the inner boundary of the habitable zone. We find the magma ocean stage can last up to 50 Myr, remove 8 Earth-oceans of water, and leave up to 2000 bars of oxygen in the atmosphere. However, if the planet forms with a hydrogen envelope as small as 0.1 Earth-masses, no water will be lost during the star's pre-main sequence phase from thermal escape processes. We find that the planets are unlikely to be in a 3:1 mean motion resonance and that both planets tidally circularize within 0.5 Gyr when tidal dissipation is held constant. However, if tidal dissipation is a function of mantle temperature and rheology, then we find that planet c's orbit may require more than 7 Gyr to circularize, during which time tidal heating may reach hundreds of terawatts. We thus conclude that the habitability of planet c depends most strongly on the initial volatile content and internal properties, but no data yet preclude the viability of an active biosphere on the planet.

astro-ph.EP↗

arXiv2023

The Semantic Scholar Open Data Platform

The volume of scientific output is creating an urgent need for automated tools to help scientists keep up with developments in their field. Semantic Scholar (S2) is an open data platform and website aimed at accelerating science by helping scholars discover and understand scientific literature. We combine public and proprietary data sources using state-of-the-art techniques for scholarly PDF content extraction and automatic knowledge graph construction to build the Semantic Scholar Academic Graph, the largest open scientific literature graph to-date, with 200M+ papers, 80M+ authors, 550M+ paper-authorship edges, and 2.4B+ citation edges. The graph includes advanced semantic features such as structurally parsed text, natural language summaries, and vector embeddings. In this paper, we describe the components of the S2 data processing pipeline and the associated APIs offered by the platform. We will update this living document to reflect changes as we add new data offerings and improve existing services.

cs.DL↗

arXiv2019

Near-Earth Supernova Explosions: Evidence, Implications, and Opportunities

There is now solid experimental evidence of at least one supernova explosion within 100 pc of Earth within the last few million years, from measurements of the short-lived isotope 60Fe in widespread deep-ocean samples, as well as in the lunar regolith and cosmic rays. This is the first established example of a specific dated astrophysical event outside the Solar System having a measurable impact on the Earth, offering new probes of stellar evolution, nuclear astrophysics, the astrophysics of the solar neighborhood, cosmic-ray sources and acceleration, multi-messenger astronomy, and astrobiology. Interdisciplinary connections reach broadly to include heliophysics, geology, and evolutionary biology. Objectives for the future include pinning down the nature and location of the established near-Earth supernova explosions, seeking evidence for others, and searching for other short-lived isotopes such as 26Al and 244Pu. The unique information provided by geological and lunar detections of radioactive 60Fe to assess nearby supernova explosions make now a compelling time for the astronomy community to advocate for supporting multi-disciplinary, cross-cutting research programs.

astro-ph.SR↗

arXiv2015

How important are electron beams in driving chromospheric evaporation in the 2014 March 29 flare?

We present high spatial resolution observations of chromospheric evaporation in the flare SOL2014-03-29T17:48. Interface Region Imaging Spectrograph (IRIS) observations of the FeXXI 1354.1 A line indicate evaporating plasma at a temperature of 10 MK along the flare ribbon during the flare peak and several minutes into the decay phase with upflow velocities between 30 km s$^{-1}$ and 200 km s$^{-1}$. Hard X-ray (HXR) footpoints were observed by RHESSI for two minutes during the peak of the flare. Their locations coincided with the locations of the upflows in parts of the southern flare ribbon but the HXR footpoint source preceded the observation of upflows in FeXXI by 30-75 seconds. However, in other parts of the southern ribbon and in the northern ribbon the observed upflows were not coincident with a HXR source in time nor space, most prominently during the decay phase. In this case evaporation is likely caused by energy input via a conductive flux that is established between the hot (25 MK) coronal source, which is present during the whole observed time-interval, and the chromosphere. The presented observations suggest that conduction may drive evaporation not only during the decay phase but also during the flare peak. Electron beam heating may only play a role in driving evaporation during the initial phases of the flare.

astro-ph.SR↗