Recent observations suggest that some type Ia supernovae ( SNe Ia ) originate from the merging of two carbon-oxygen white dwarfs ( CO WDs ) .
Meanwhile , recent hydrodynamical simulations have indicated that the accretion-induced collapse may be avoided under certain conditions when double WDs merge violently .
However , the properties of SNe Ia from this violent merger scenario are highly dependent on a particular mass-accretion stage , the so-called WD + He subgiant channel , during which the primary WD is able to increase its mass by accreting He-rich material from a He subgiant before the systems evolves into a double WD system .
In this article , we aim to study this particular evolutionary stage systematically and give the properties of violent WD mergers .
By employing the Eggleton stellar evolution code , we followed a large number of binary calculations and obtained the regions in parameter space for producing violent mergers based on the WD + He subgiant channel .
According to these simulations , we found that the primary WDs can increase their mass by \sim 0.10 - 0.45 M _ { \odot } during the mass-accretion stage .
We then conducted a series of binary population synthesis calculations and found that the Galactic SN Ia birthrate from this channel is about 0.01 - 0.4 \times 10 ^ { -3 } yr ^ { -1 } .
This suggests that the violent WD mergers from this channel may only contribute to \sim 0.3 \% - 10 \% of all SNe Ia in our Galaxy .
The delay times of violent WD mergers from this channel are \geq 1.7 Gyr , contributing to the SNe Ia in old populations .
We also found that the WD + He subgiant channel is the dominent way for producing violent WD mergers that may be able to eventually explode as SNe Ia .