LDN 1641

Lore & Background

LDN 1641 is a dark cloud that forms part of the Orion A molecular cloud. At its northern end it is connected to the Orion Nebula, and at its southern end it is connected to the dark cloud LDN 1647. The cloud contains more than a thousand young stellar objects (YSOs). It is a relatively low-density star-forming region without any massive O- or B-type stars that could disturb star formation with ultraviolet radiation, providing a contrasting environment to the Orion Nebula.

Star formation in Lynds 1641 began about 2-3 million years ago. About 50% of its YSOs have circumstellar disks, many of which were directly imaged with ALMA. Among these, a large fraction (21%) are transitional disks—disks with a gap. The disks appear much smaller in ALMA images than those in closer star-forming regions, such as the Scorpius-Centaurus association, because LDN 1641 is about four times more distant.

The cloud exhibits a strong distance gradient. VLBA observations place the southern part at around 428±10 parsecs and the Orion Nebula at 388±5 parsecs. Gaia data show a trend toward a closer distance at the northern part, with about 453 parsecs at the southern end and about 383 parsecs at the northern end. Other nebulae overlap with Lynds 1641, including NGC 1999, IC 427/428, IC 429/430, HH34, and HH 1/2.

Reader's Guide

LDN 1641 is significant as a nearby laboratory for studying star formation in a quiescent environment, free from the disruptive ultraviolet radiation of massive stars. Its large population of over a thousand young stellar objects, many with circumstellar disks, allows researchers to examine disk evolution and planet formation processes. The high fraction of transitional disks (21%) is particularly notable, as these gaps may indicate ongoing planet formation. The cloud's distance gradient, measured by both VLBA and Gaia, provides important constraints on the three-dimensional structure of the Orion A molecular cloud. Because the disks in LDN 1641 appear smaller in ALMA images compared to those in closer regions like Scorpius-Centaurus, observations here complement studies of nearer star-forming sites. The overlap with other nebulae such as NGC 1999 and Herbig-Haro objects further enriches the region as a target for multi-wavelength studies. Overall, LDN 1641 offers a unique perspective on star formation in a low-mass, undisturbed setting, contrasting with the more violent Orion Nebula.

Did You Know?

• About 50% of its young stellar objects have circumstellar disks, many imaged with ALMA.
• 21% of the disks are transitional disks, meaning they have a gap.
• The cloud has a strong distance gradient, from about 383 parsecs at the north to about 453 parsecs at the south.