That lens is very unique. I doubt you will even find any other zoom faster than f2.6. And there are very few affordable f2.8 zoom lenses.
The closest Pentax zooms I can think of are Pentax
DA 12-24mm f4,
DA 16-45mm f4 and
FA 20-35mm f4. The latter two are discontinued, but can be found used. Third party, you have Sigma 10-20mm,
Tamron 10-20mm.. or even the Tamron 17-50mm f2.8, if 17mm is wide enough.
And then there are Samyang primes, which are probably the best choice for astrophotography, but have no zoom, no AF. Then again, the Samyang 10mm has such a wide depth of field, you don't need to focus much if you use f8. Same thing for Samyang 14mm, but this one needs some focusing. For everyday photography I would recommend the Samyang 16mm f2.0, though, as its not as awkwardly wide.