Remnant Population — Elizabeth Moon

ALONE AT LAST

People had always told Ofelia what to do; for once she was going to do what she wanted. She refused to get on the cryo ships, refused to leave the only world she could call home. And when they finally came for her, she hid--not that the authorities looked all that hard for one crazy old woman. Now Ofelia is alone, content to live with no more demands on her self or her time, the only remaining settler on an abandoned planet.

Then new settlers arrive. At first she fears they have come to reoccupy the settlement she has come to think of as hers--but they land far away. And as Ofelia secretly listens, they are slaughtered to the last child by stone-age aliens no one knew were there.

Now it is up to Ofelia to save the aliens from Earth's wrath... .