The scatter of the M dwarf mass–radius relationship Parsons, S G; Gänsicke, B T; Marsh, T R ...
Monthly notices of the Royal Astronomical Society,
11/2018, Letnik:
481, Številka:
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M dwarfs are prime targets in the hunt for habitable worlds around other stars. This
is due to their abundance as well as their small radii and low masses and tempera-
tures, which facilitate the ...detection of temperate, rocky planets in orbit around them.
However, the fundamental properties of M dwarfs are difficult to constrain, often lim-
iting our ability to characterise the planets they host. Here we test several theoretical
relationships for M dwarfs by measuring 23 high precision, model-independent masses
and radii for M dwarfs in binaries with white dwarfs. We find a large scatter in the
radii of these low-mass stars, with 25 per cent having radii consistent with theoret-
ical models while the rest are up to 12 per cent over-inflated. This scatter is seen
in both partially- and fully-convective M dwarfs. No clear trend is seen between the
over-inflation and age or metallicity, but there are indications that the radii of slowly
rotating M dwarfs are more consistent with predictions, albeit with a similar amount of
scatter in the measurements compared to more rapidly rotating M dwarfs. The sample
of M dwarfs in close binaries with white dwarfs appears indistinguishable from other
M dwarf samples, implying that common envelope evolution has a negligible impact on
their structure. We conclude that theoretical and empirical mass-radius relationships
lack the precision and accuracy required to measure the fundamental parameters of
M dwarfs well enough to determine the internal structure and bulk composition of the
planets they host.
Peer Reviewed
The broadening of atomic emission lines by high-velocity motion of gas near accreting supermassive black holes is an observational hallmark of quasars
. Observations of broad emission lines could ...potentially constrain the mechanism for transporting gas inwards through accretion disks or outwards through winds
. The size of regions for which broad emission lines are observed (broad-line regions) has been estimated by measuring the delay in light travel time between the variable brightness of the accretion disk continuum and the emission lines
-a method known as reverberation mapping. In some models the emission lines arise from a continuous outflow
, whereas in others they arise from orbiting gas clouds
. Directly imaging such regions has not hitherto been possible because of their small angular size (less than 10
arcseconds
). Here we report a spatial offset (with a spatial resolution of 10
arcseconds, or about 0.03 parsecs for a distance of 550 million parsecs) between the red and blue photo-centres of the broad Paschen-α line of the quasar 3C 273 perpendicular to the direction of its radio jet. This spatial offset corresponds to a gradient in the velocity of the gas and thus implies that the gas is orbiting the central supermassive black hole. The data are well fitted by a broad-line-region model of a thick disk of gravitationally bound material orbiting a black hole of 3 × 10
solar masses. We infer a disk radius of 150 light days; a radius of 100-400 light days was found previously using reverberation mapping
. The rotation axis of the disk aligns in inclination and position angle with the radio jet. Our results support the methods that are often used to estimate the masses of accreting supermassive black holes and to study their evolution over cosmic time.
Context. Apparent period variations detected in several eclipsing, close-compact binaries are frequently interpreted as being caused by circumbinary giant planets. This interpretation raises the ...question of the origin of the potential planets that must have either formed in the primordial circumbinary disk, together with the host binary star, and survived its evolution into a close-compact binary or formed in a post-common-envelope circumbinary disk that remained bound to the post-common-envelope binary (PCEB). Aims. Here we combine current knowledge of planet formation and the statistics of giant planets around primordial and evolved binary stars with the theory of close-compact binary star evolution aiming to derive new constraints on possible formation scenarios. Methods. We compiled a comprehensive list of observed eclipsing PCEBs, estimated the fraction of systems showing apparent period variations, reconstructed the evolutionary history of the PCEBs, and performed binary population models of PCEBs to characterize their main sequence binary progenitors. We reviewed the currently available constraints on the fraction of PCEB progenitors that host circumbinary giant planets. Results. We find that the progenitors of PCEBs are very unlikely to be frequent hosts of giant planets (≲10 per cent), while the frequency of PCEBs with observed apparent period variations is very high (~90 per cent). Conclusions. The variations in eclipse timings measured in eclipsing PCEBs are probably not caused by first-generation planets that survived common-envelope evolution. The remaining options for explaining the observed period variations are second-generation planet formation or perhaps variations in the shape of a magnetically active secondary star. We suggest observational tests for both options.
We present high-precision, model-independent, mass and radius measurements for 16 white dwarfs in detached eclipsing binaries and combine these with previously published data to test the theoretical ...white dwarf mass–radius relationship. We reach a mean precision of 2.4 per cent in mass and 2.7 per cent in radius, with our best measurements reaching a precision of 0.3 per cent in mass and 0.5 per cent in radius. We find excellent agreement between the measured and predicted radii across a wide range of masses and temperatures. We also find the radii of all white dwarfs with masses less than 0.48 M¿ to be fully consistent with helium core models, but they are on average 9 per cent larger than those of carbon–oxygen core models. In contrast, white dwarfs with masses larger than 0.52 M¿ all have radii consistent with carbon–oxygen core models. Moreover, we find that all but one of the white dwarfs in our sample have radii consistent with possessing thick surface hydrogen envelopes (10-5 = MH/MWD = 10-4), implying that the surface hydrogen layers of these white dwarfs are not obviously affected by common envelope evolution.
Peer Reviewed
Abstract
Low-mass helium-core white dwarfs (WDs) with masses below
$0.5\,{\mathrm{M_{{\odot}}}$
are known to be formed in binary star systems but unexpectedly, a significant fraction of them seems to ...be single. On the other hand, in cataclysmic variables (CVs), a large number of low-mass WD primary stars is predicted but not observed. We recently showed that the latter problem can be solved if consequential angular momentum loss causes especially CVs with low-mass WDs to merge and form single stars. Here we simulate the population of single WDs resulting from single-star evolution and from binary star mergers taking into account these new merging CVs. We show that according to the revised model of CV evolution, merging CVs might be the dominant channel leading to the formation of low-mass single WDs and that the predicted relative numbers are consistent with observations. This can be interpreted as further evidence for the revised model of CV evolution we recently suggested. This model includes consequential angular momentum loss that increases with decreasing WD mass and might not only explain the absence of low-mass WD primaries in CVs but also the existence of single low-mass WDs.
Abstract
We present HST spectroscopy for 45 cataclysmic variables (CVs), observed with HST/COS and HST/STIS. For 36 CVs, the white dwarf is recognisable through its broad Ly α absorption profile and ...we measure the white dwarf effective temperatures (T
eff) by fitting the HST data assuming log g = 8.35, which corresponds to the average mass for CV white dwarfs (≃0.8 M⊙). Our results nearly double the number of CV white dwarfs with an accurate temperature measurement. We find that CVs above the period gap have, on average, higher temperatures (〈T
eff〉 ≃ 23 000 K) and exhibit much more scatter compared to those below the gap (〈T
eff〉 ≃ 15 000 K). While this behaviour broadly agrees with theoretical predictions, some discrepancies are present: (i) all our new measurements above the gap are characterized by lower temperatures (T
eff ≃ 16 000–26 000 K) than predicted by the present-day CV population models (T
eff ≃ 38 000–43 000 K); (ii) our results below the gap are not clustered in the predicted narrow track and exhibit in particular a relatively large spread near the period minimum, which may point to some shortcomings in the CV evolutionary models. Finally, in the standard model of CV evolution, reaching the minimum period, CVs are expected to evolve back towards longer periods with mean accretion rates
$\dot{M}\lesssim 2 \times 10^{-11}\,\mathrm{M}_{\odot }\,\mathrm{yr}^{-1}$
, corresponding to T
eff ≲ 11 500 K. We do not unambiguously identify any such system in our survey, suggesting that this major component of the predicted CV population still remains elusive to observations.
Malnutrition affects up to one billion people in the world and is a major cause of mortality. In many cases, malnutrition is associated with diarrhoea and intestinal inflammation, further ...contributing to morbidity and death. The mechanisms by which unbalanced dietary nutrients affect intestinal homeostasis are largely unknown. Here we report that deficiency in murine angiotensin I converting enzyme (peptidyl-dipeptidase A) 2 (Ace2), which encodes a key regulatory enzyme of the renin-angiotensin system (RAS), results in highly increased susceptibility to intestinal inflammation induced by epithelial damage. The RAS is known to be involved in acute lung failure, cardiovascular functions and SARS infections. Mechanistically, ACE2 has a RAS-independent function, regulating intestinal amino acid homeostasis, expression of antimicrobial peptides, and the ecology of the gut microbiome. Transplantation of the altered microbiota from Ace2 mutant mice into germ-free wild-type hosts was able to transmit the increased propensity to develop severe colitis. ACE2-dependent changes in epithelial immunity and the gut microbiota can be directly regulated by the dietary amino acid tryptophan. Our results identify ACE2 as a key regulator of dietary amino acid homeostasis, innate immunity, gut microbial ecology, and transmissible susceptibility to colitis. These results provide a molecular explanation for how amino acid malnutrition can cause intestinal inflammation and diarrhoea.
A central hypothesis in the theory of cataclysmic variable (CV) evolution is the need to explain the observed lack of accreting systems in the ≃2–3 h orbital period range, known as the period gap. ...The standard model, disrupted magnetic braking (DMB), reproduces the gap by postulating that CVs transform into inconspicuous detached white dwarf (WD) plus main sequence systems, which no longer resemble CVs. However, observational evidence for this standard model is currently indirect and thus this scenario has attracted some criticism throughout the last decades. Here, we perform a simple but exceptionally strong test of the existence of detached CVs (dCVs). If the theory is correct, dCVs should produce a peak in the orbital period distribution of detached close binaries consisting of a WD and an M4–M6 secondary star. We measured six new periods which brings the sample of such binaries with known periods below 10 h to 52 systems. An increase of systems in the ≃2–3 h orbital period range is observed. Comparing this result with binary population models, we find that the observed peak cannot be reproduced by post-common envelope binaries (PCEBs) alone and that the existence of dCVs is needed to reproduce the observations. Also, the WD mass distribution in the gap shows evidence of two populations in this period range, i.e. PCEBs and more massive dCVs, which is not observed at longer periods. We therefore conclude that CVs are indeed crossing the gap as detached systems, which provides strong support for the DMB theory.
White dwarfs are compact stars, similar in size to Earth but approximately 200,000 times more massive. Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, ...but when in close orbits with less dense stars, white dwarfs can strip material from their companions and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies. Here we report the discovery of a white dwarf/cool star binary that emits from X-ray to radio wavelengths. The star, AR Scorpii (henceforth AR Sco), was classified in the early 1970s as a δ-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56-hour period close binary, pulsing in brightness on a period of 1.97 minutes. The pulses are so intense that AR Sco's optical flux can increase by a factor of four within 30 seconds, and they are also detectable at radio frequencies. They reflect the spin of a magnetic white dwarf, which we find to be slowing down on a 10
-year timescale. The spin-down power is an order of magnitude larger than that seen in electromagnetic radiation, which, together with an absence of obvious signs of accretion, suggests that AR Sco is primarily spin-powered. Although the pulsations are driven by the white dwarf's spin, they mainly originate from the cool star. AR Sco's broadband spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf's magnetosphere.
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