Star formation in the filament of S254-S258 OB complex: a cluster in the
  process of making
  Infrared Dark Clouds are ideal laboratories to study the initial processes of
high-mass star and star cluster formation. We investigated star formation
activity of an unexplored filamentary dark cloud (~5.7pc x 1.9pc), which itself
is part of a large filament (~20pc) located in the S254-S258 OB complex at a
distance of 2.5kpc. Using MIPS Spitzer 24 micron data, we uncover 49 sources
with SNR greater than 5. We identified 45 sources as candidate YSOs of Class I,
Flat-spectrum & Class II nature. Additional 17 candidate YSOs (9 Class I & 8
Class II) are also identified using JHK and WISE photometry. We find that the
protostar to Class II sources ratio (~2) and the protostar fraction (~70%) of
the region are high. When the protostar fraction compared to other young
clusters, it suggests that the star formation in the dark cloud was possibly
started only 1 Myr ago. Combining the NIR photometry of the YSO candidates with
the theoretical evolutionary models, we infer that most of the candidate YSOs
formed in the dark cloud are low-mass (<2 Msolar) in nature. We examine the
spatial distribution of the YSOs and find that majority of them are linearly
aligned along the highest column density line (N(H2) ~1 x 10^22 cm^-2) of the
dark cloud along its long axis at mean nearest neighbor separation of ~0.2pc.
Using observed properties of the YSOs, physical conditions of the cloud and a
simple cylindrical model, we explore the possible star formation process of
this filamentary dark cloud and suggest that gravitational fragmentation within
the filament should have played a dominant role in the formation of the YSOs.
From the total mass of the YSOs, gaseous mass associated with the dark cloud,
and surrounding environment, we infer that the region is presently forming
stars at an efficiency ~3% and a rate ~30 Msolar Myr^-1, and may emerge to a
richer cluster.
