Hi jstevenot,
To force a link to open in a new tab you need to add the target=”_blank” attribute to the link. As far as I know, it’s recommended that you let users decide whether they want to open a link in a new tab or not, however.
In any case, to do it, you could create a child theme for Twenty Twelve and duplicate the content.php file from the parent. Find this line of code:
<h1 class="entry-title">
<a href="<?php the_permalink(); ?>" title="<?php echo esc_attr( sprintf( __( 'Permalink to %s', 'twentytwelve' ), the_title_attribute( 'echo=0' ) ) ); ?>" rel="bookmark"><?php the_title(); ?></a>
</h1>
Modify it by adding the target attribute to the link, like so:
<h1 class="entry-title">
<a href="<?php the_permalink(); ?>" target="_blank" title="<?php echo esc_attr( sprintf( __( 'Permalink to %s', 'twentytwelve' ), the_title_attribute( 'echo=0' ) ) ); ?>" rel="bookmark"><?php the_title(); ?></a>
</h1>
This theme has other content templates. Check if any of these need the same treatment. For instance, content-aside.php has a linked title.
I hope this helps.