Flight Crew Air Transport Pilot (ATP)
Distraction; Situational Awareness; Communication Breakdown; Confusion
Conflict Airborne Conflict; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action
Company Policy; Procedure
During a night time flight under IFR in VFR conditions to Boston; we were descending via the QUABN3 arrival.  In the vicinity of UROWT intersection northwest of Boston; we listened to a conversation between Boston Approach and a VFR 'Research' Aircraft Y that was loitering VFR on the STAR corridor at 15;500 feet.  ATC was attempting to move the aircraft out of the arrival area for traffic but Aircraft Y requested to stay on that station for the next hour.  ATC then vectored an aircraft directly in front of us 30 degrees left of course to avoid Aircraft Y.  We anticipated that we would also be vectored off the arrival and shortly after; we were given vectors 30 degrees left of course and advised of the traffic at 15;500 Feet.  We were at approximately 17;000 Feet descending in order to meet the altitude restrictions of the STAR.  Without warning or advising ATC; Aircraft Y turned directly toward us which rapidly triggered a TCAS Resolution Advisory for us to CLIMB.  We immediately reversed the descent and began a climb and advised ATC that we were responding to an resolution advisory (RA).  We observed a 300 foot altitude difference between us and the RED TCAS icon at the closest proximity.  Lateral proximity appeared to be about 1 mile.  The 'Clear of Conflict' message was not heard until we had climbed well above the traffic; to approximately FL180.  We then were re-cleared to resume the arrival although we were unable to descend in time to meet at least one of the at or below altitude restrictions.  Once re-established on the arrival; we landed uneventfully. Suggestions1.  Although we anticipated a vector away from the traffic; an altitude re-clearance above the conflict and release from altitude descent restriction by ATC could have been an easier; faster; and safer way to resolve the traffic conflict.2.  Although ATC is always trying to accommodate VFR traffic requests; in my view the request by the 'Research' Aircraft Y should have been denied until ATC was able to better coordinate a long-term plan for the Aircraft Y's lengthy request to remain on that station and should have given the VFR traffics instructions to advise all changes in altitude or heading.
A Corporate Captain reported taking evasive action from a VFR Small Transport; High Wing; 2 Turboprop Engine near 15;500 Feet while descending on the QUABN3 arrival to BOS at night after ATC allowed the aircraft to remain in the arrival corridor.