The progenitors of type Ia supernovae in the semidetached binaries with red giant donors

Context. The companions of the exploding carbon-oxygen white dwarfs (CO WDs) for producing type Ia supernovae (SNe Ia) are still not conclusively confirmed. A red-giant (RG) star has been suggested to be the mass donor of the exploding WD, named as the symbiotic channel. However, previous studies on the this channel gave a relatively low rate of SNe Ia. Aims. We aim to systematically investigate the parameter space, Galactic rates and delay time distributions of SNe Ia from the symbiotic channel by employing a revised mass-transfer prescription. Methods. We adopted an integrated mass-transfer prescription to calculate the mass-transfer process from a RG star onto the WD. In this prescription, the mass-transfer rate varies with the local material states. Results. We evolved a large number of WD+RG systems, and found that the parameter space of WD+RG systems for producing SNe Ia is significantly enlarged. This channel could produce SNe Ia with intermediate and old ages, contributing to at most 5% of all SNe Ia in the Galaxy. Our model increases the SN Ia rate from this channel by a factor of 5. We suggest that the symbiotic systems RS Oph and T CrB are strong candidates for the progenitors of SNe Ia.Comment: 8 pages, 6 figure

Deletion of <i>BDF1</i> reduced <i>HAL2</i> expression at both mRNA and protein levels.

<p>The mid-log phase cultures in YPD at 30°C were incubated for 45 min with or without 0.5 mol.L⁻¹ NaCl. (A) Relative fold-changes of <i>HAL2</i> mRNA were calculated against the wild type without NaCl treatment. Error bars denote standard deviation (SD). **P<0.01 vs. wild type under the same treatment, ^# P<0.01 vs. <i>bdf1Δ</i> under the same treatment, n = 3. (B) The Hal2p protein level was analyzed with whole cell protein by Western blot. β-tubulin was used as control. The polyclonal Hal2p antibody and anti-β-tubulin antisera were used in Western blot analysis.</p

Overexpression of <i>HAL2</i> in <i>bdf1Δ</i> recovered its resistance to NaCl.

<p>5 µl aliquots of 10-fold serial dilutions of the mid-log phase cultures were spotted onto YPD plates and incubated at 30°C for 3 d.</p

<i>HAL2</i> overexpression affected ROS accumulation and partially affected the mitochondrial function.

<p>Mid-log phase cells were incubated for 45 min with or without 0.5 mol.L⁻¹ NaCl. ROS were detected by dihydrorhodamine 123. Mitochondrial membrane potential was measured with JC-1 (A) The ROS (B) and <i>Δ</i>φ (C) fluorescence values were quantified as the relative fluorescence intensity per strain by ImageJ software and averaged from ∼50 cells. Error bars denote standard deviation (SD). *P<0.05, **P<0.01 vs. strains without <i>HAL2</i> overexpression under the same treatment, n = 4. (D) 5 ul aliquots of 10-fold serial dilutions of the mid-log phage cultures were spotted onto YPG plates and incubated at 30°C for 3 d.</p

The intracellular Na+ concentration in <i>bdf1Δ</i> was lower than that in wild type.

<p>Mid-log phase cells were grown for 5 h with or without 0.5 mol.L⁻¹ NaCl. The treated cells were washed with MgCl₂ and air dried. The dried cells were nitrified with nitric acid. The Na⁺ concentration was analyzed by atomic absorption spectrophotometry at 589 nm. Error bars denote standard deviation (SD). *P<0.05, **P<0.01 vs. wild type under the same treatment, ^# P<0.01 vs. <i>ena1Δ</i> under the same treatment, n = 3.</p

Salt sensitivity of <i>bdf1Δ</i> was not due to intracellular pAp or the lack of methionine.

<p>Cells grown to OD₆₀₀ = 0.2∼0.4 in SD medium were incubated at 30°C without or with different concentrations of NaCl for 4 h, or 0.1 mol.L⁻¹ LiCl for 2 h. The intracellular pAp concentration was determined as described in Materials and Methods (A–E). 5 ul aliquots of 10-fold serial dilutions of the mid-log phase cultures were spotted onto plates and incubated at 30°C for 3 d (F, G).</p

Bdf1p regulated <i>HAL2</i> expression indirectly.

<p>The anti-Flag antibody was used for Bdf1p precipitation and IgG was used as a negative control. The promoter regions probed by ChIP correspond to nucleotides −200 to −1. Error bars denote standard deviation (SD). *P<0.05, **P<0.01 vs. wild type under the same treatment, n = 3.</p

Strains and plasmids used in this study.

<p>Strains and plasmids used in this study.</p

Hal2p Functions in Bdf1p-Involved Salt Stress Response in <i>Saccharomyces cerevisiae</i>

<div><p>The <i>Saccharomyces cerevisiae</i> Bdf1p associates with the basal transcription complexes TFIID and acts as a transcriptional regulator. Lack of Bdf1p is salt sensitive and displays abnormal mitochondrial function. The nucleotidase Hal2p detoxifies the toxic compound 3′ -phosphoadenosine-5′-phosphate (pAp), which blocks the biosynthesis of methionine. Hal2p is also a target of high concentration of Na⁺. Here, we reported that <i>HAL2</i> overexpression recovered the salt stress sensitivity of <i>bdf1Δ</i>. Further evidence demonstrated that <i>HAL2</i> expression was regulated indirectly by Bdf1p. The salt stress response mechanisms mediated by Bdf1p and Hal2p were different. Unlike <i>hal2Δ</i>, high Na⁺ or Li⁺ stress did not cause pAp accumulation in <i>bdf1Δ</i> and methionine supplementation did not recover its salt sensitivity. <i>HAL2</i> overexpression in <i>bdf1Δ</i> reduced ROS level and improved mitochondrial function, but not respiration. Further analyses suggested that autophagy was apparently defective in <i>bdf1Δ</i>, and autophagy stimulated by Hal2p may play an important role in recovering mitochondrial functions and Na⁺ sensitivity of <i>bdf1Δ</i>. Our findings shed new light towards our understanding about the molecular mechanism of Bdf1p-involved salt stress response in budding yeast.</p></div