X-ray emission of brown dwarfs: towards constraining the dependence on age, luminosity, and temperature
Bibliographic record
Abstract
Aims.We observed brown dwarfs in different evolutionary stages with the Chandra X-ray Observatory with the aim to disentangle the influence of different stellar parameters on the X-ray emission of substellar objects. The ages of our three targets (HR 7329 B, Gl 569 Bab, and HD 130948 BC) are constrained by them being companions to main-sequence stars of known age. With both known age and effective temperature or bolometric luminosity, the mass can be derived from evolutionary models. Methods.Combining the new observations with previous studies presented in the literature yields a brown dwarf sample that covers the age range from ~1 Myr to ~1 Gyr. Since the atmospheric temperature of brown dwarfs is approximately constant at young ages, a sample with a large age spread is essential for investigating the possible influence of effective temperature on X-ray activity. Results.Two out of three brown dwarfs are detected with Chandra, with variable lightcurves and comparatively soft spectra. Combining our results with published data allows us to consider a subsample of high-mass brown dwarfs (with 0.05-0.07 ), thus eliminating mass from the list of free parameters. We find evidence that X-ray luminosity declines with decreasing bolometric luminosity steeper than expected from the canonical relation for late-type stars (). Effective temperature is identified as a likely parameter responsible for the additional decline of X-ray activity in the more evolved (and therefore cooler) brown dwarfs of the “high-mass” sample. In another subsample of brown dwarfs characterized by similar effective temperature, the X-ray luminosity scales with the bolometric luminosity without indications for a deviation from the canonical range of 10-3...-5 observed for late-type stars.Conclusions.Our findings support the idea that effective temperature plays a critical role for the X-ray activity in brown dwarfs. This underlines an earlier suggestion based on observations of chromospheric Hα emission in ultracool dwarfs that the low ionization fraction in the cool brown dwarf atmospheres may suppress magnetic activity.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".