18 research outputs found
Searching for a Companion Star of Tycho's Type Ia Supernova with Optical Spectroscopic Observations
We report our first results of photometric and spectroscopic observations for
Tycho's supernova remnant (SNR Tycho) to search for the companion star of a
type Ia supernova (SN Ia). From photometric observations using Suprime-Cam on
the Subaru Telescope, we have picked up stars brighter than 22 mag (in
-band) for spectroscopy, which are located within a circular region with the
radius of 30" around the center of SNR Tycho. If the ejecta of young supernova
remnants, such as SNR Tycho, have a sufficient amount of Fe I, we should be
able to detect absorption lines at 3720 \AA and 3860 \AA associated with
transitions from the ground state of Fe I in the spectrum of the companion
star. To identify the companion star of a SN Ia using these characteristic
absorption lines of Fe I, we made optical low-resolution spectroscopy of their
targets using FOCAS on the Subaru Telescope. In our spectroscopic observations,
we obtained spectra of 17 stars in the SNR Tycho region and compare them with
template stellar spectra. We detect significant absorption lines from two stars
at 3720 \AA. Since widths of their absorption lines are broad, it is likely
that the detected absorptions are due to Fe I in the expanding ejecta of SNR
Tycho. However, none of stars exhibits a clear red wing in the observed
profiles of the absorption, though a star in the background of the SNR should
show it. Hence, we suggest another interpretation that the detected absorption
lines might be caused by the peculiarity of stars. A star named Tycho(E) has
the absorption line at 3720 \AA and its projected position is close to the
center of SNR Tycho. Based on our observations, Tycho(E) is a new candidate as
the companion star of Tycho's supernova.Comment: 17 pages, 28 figures, accepted for publication in PAS
The Stellar Abundances for Galactic Archaeology (SAGA) Database II - Implications for Mixing and Nucleosynthesis in Extremely Metal-Poor Stars and Chemical Enrichment of the Galaxy
We discuss the characteristics of known extremely metal-poor (EMP) stars in
the Galaxy using the Stellar Abundances for Galactic Archaeology (SAGA)
database (Suda et al. 2008, PASJ, 60, 1159).The analyses of carbon-enhanced
stars in our sample suggest that the nucleosynthesis in AGB stars can
contribute to the carbon enrichment in a different way depending on whether the
metallicity is above or below [Fe/H] ~ -2.5, which is consistent with the
current models of stellar evolution at low metallicity. We find the transition
of the initial mass function at [Fe/H] ~ -2 in the viewpoint of the
distribution of carbon abundance and the frequency of carbon-enhanced stars.
For observed EMP stars, we confirmed that some, not all, of observed stars
might have undergone at least two types of extra mixing to change their surface
abundances. One is to deplete the lithium abundance during the early phase of
red giant branch. Another is to decrease the C/N ratio by one order of
magnitude during the red giant branch phase. Observed small scatters of
abundances for alpha-elements and iron-group elements suggest that the chemical
enrichment of our Galaxy takes place in a well-mixed interstellar medium. We
find that the abundance trends of alpha-elements are highly correlated with
each other, while the abundances of iron-group elements are subject to
different slopes relative to the iron abundance. This implies that the
supernova yields of alpha-elements are almost independent of mass and
metallicity, while those of iron-group elements have a metallicity dependence
or mass dependence with the variable initial mass function.The occurrence of
the hot bottom burning in the mass range of 5 <~ M / Msun <~ 6 is consistent
with the initial mass function of the Galaxy peaked at ~ 10 - 12 Msun to be
compatible with the statistics of carbon enhanced stars with and without
s-process element (truncated)Comment: 35 pages, 27 figures, 6 tables, accepted by MNRAS, database to
reproduce figures is available at http://saga.sci.hokudai.ac.j
The Origin of Carbon-Enhancement and Initial Mass Function of Extremely Metal-Poor Stars in the Galactic Halo
It is known that the carbon-enhanced, extremely metal-poor (CEMP) stars
constitute a substantial proportion in the extremely metal-poor (EMP) stars of
the Galactic Halo, by far larger than CH stars in Population II stars. We
investigate their origin with taking into account an additional evolutionary
path to the surface carbon-enrichment, triggered by hydrogen engulfment by the
helium flash convection, in EMP stars of . This process
is distinct from the third dredge-up operating in more metal-rich stars and
also in EMP stars. In binary systems of EMP stars, the secondary stars become
CEMP stars through mass transfer from the primary stars of low and intermediate
masses, which have developed the surface carbon-enhancement. Our binary
scenario can predict the variations in the abundances not only for carbon but
also for nitrogen and s-process elements and reasonably explain the observed
properties such as the stellar distributions with respect to the carbon
abundances, the binary periods, and the evolutionary stages. Furthermore, from
the observed frequencies of CEMP stars with and without s-process element
enhancement, we demonstrate that the initial mass function of EMP stars need to
give the mean mass ~10\msun under the reasonable assumptions on the
distributions of orbital separations and mass ratio of binary components. This
also indicates that the currently observed EMP stars were exclusively born as
the secondary members of binaries, making up remnants of EMP binary
systems of mass ~10^8\msun in total; in addition to CEMP stars with white
dwarf companions, a significant fraction of them have experienced supernova
explosions of their companions. We discuss the implications of the present
results in relation to the formation of Galactic halo.Comment: 66 pages, 12 figures, 2 tables Accepted for publication in Ap
Ablation of TSC2 Enhances Insulin Secretion by Increasing the Number of Mitochondria through Activation of mTORC1
) mice. The present study examines the effects of TSC2 ablation on insulin secretion from pancreatic beta cells. mice and TSC2 knockdown insulin 1 (INS-1) insulinoma cells treated with small interfering ribonucleic acid were used to investigate insulin secretion, ATP content and the expression of mitochondrial genes. mice exhibit hyperinsulinemia due to an increase in the number of mitochondria as well as enlargement of individual beta cells via activation of mTORC1.Activation of mTORC1 by TSC2 ablation increases mitochondrial biogenesis and enhances insulin secretion from pancreatic beta cells
A case of pituitary metastasis discovered when diabetes insipidus developed in a patient 20 years after breast cancer treatment
The patient was a 52-year-old woman. She had a history of left breast cancer at age 32 years, with no recurrences. She was examined for a feeling of oral dryness and nocturia, and central diabetes insipidus was diagnosed. A mass was seen in the posterior pituitary on magnetic resonance imaging, and multiple pulmonary nodules were seen on computed tomography. Breast cancer metastases were diagnosed in both tissues. Since this patient had no cancer other than the breast cancer treated 20 years earlier, it was difficult to reach a diagnosis of pituitary metastasis with pituitary gland imaging alone. In estrogen receptor-positive breast cancer, there may be recurrences after a long period of time. It may be that recommending a full body examination could be useful in the differential diagnosis of metastasis even in patients who have had a long disease-free period, if they had undergone surgery for breast cancer
Biphasic Response of Pancreatic β-Cell Mass to Ablation of Tuberous Sclerosis Complex 2 in Mice▿
Recent studies have demonstrated the importance of insulin or insulin-like growth factor 1 (IGF-1) for regulation of pancreatic β-cell mass. Given the role of tuberous sclerosis complex 2 (TSC2) as an upstream molecule of mTOR (mammalian target of rapamycin), we examined the effect of TSC2 deficiency on β-cell function. Here, we show that mice deficient in TSC2, specifically in pancreatic β cells (βTSC2−/− mice), manifest increased IGF-1-dependent phosphorylation of p70 S6 kinase and 4E-BP1 in islets as well as an initial increased islet mass attributable in large part to increases in the sizes of individual β cells. These mice also exhibit hypoglycemia and hyperinsulinemia at young ages (4 to 28 weeks). After 40 weeks of age, however, the βTSC2−/− mice develop progressive hyperglycemia and hypoinsulinemia accompanied by a reduction in islet mass due predominantly to a decrease in the number of β cells. These results thus indicate that TSC2 regulates pancreatic β-cell mass in a biphasic manner
Ablation of C/EBPβ alleviates ER stress and pancreatic β cell failure through the GRP78 chaperone in mice
Pancreatic β cell failure is thought to underlie the progression from glucose intolerance to overt diabetes, and ER stress is implicated in such β cell dysfunction. We have now shown that the transcription factor CCAAT/enhancer-binding protein β (C/EBPβ) accumulated in the islets of diabetic animal models as a result of ER stress before the onset of hyperglycemia. Transgenic overexpression of C/EBPβ specifically in β cells of mice reduced β cell mass and lowered plasma insulin levels, resulting in the development of diabetes. Conversely, genetic ablation of C/EBPβ in the β cells of mouse models of diabetes, including Akita mice, which harbor a heterozygous mutation in Ins2 (Ins2WT/C96Y), and leptin receptor–deficient (Lepr–/–) mice, resulted in an increase in β cell mass and ameliorated hyperglycemia. The accumulation of C/EBPβ in pancreatic β cells reduced the abundance of the molecular chaperone glucose-regulated protein of 78 kDa (GRP78) as a result of suppression of the transactivation activity of the transcription factor ATF6α, thereby increasing the vulnerability of these cells to excess ER stress. Our results thus indicate that the accumulation of C/EBPβ in pancreatic β cells contributes to β cell failure in mice by enhancing susceptibility to ER stress