[LINK] "On the Nature of the Local Spiral Arm of the Milky Way"

[rfmcdonald]

A post by Universe Today's Elizabeth Howell pointed me to a recent paper suggesting that our Orion Arm--the branch of the Milky Way Galaxy where we and almost all of the stars in our night sky lie--may, well, actually be an arm and not a mere spur of another arm. From the abstract:

Trigonometric parallax measurements of nine water masers associated with the Local arm of the Milky Way were carried out as part of the BeSSeL Survey using the VLBA. When combined with 21 other parallax measurements from the literature, the data allow us to study the distribution and 3-dimensional motions of star forming regions in the spiral arm over the entire northern sky. Our results suggest that the Local arm does not have the large pitch angle characteristic of a short spur. Instead its active star formation, overall length (>5 kpc), and shallow pitch angle (~10 degrees) suggest that it is more like the adjacent Perseus and Sagittarius arms; perhaps it is a branch of one of these arms. Contrary to previous results, we find the Local arm to be closer to the Perseus than to the Sagittarius arm, suggesting that a branching from the former may be more likely. An average peculiar motion of near-zero toward both the Galactic center and north Galactic pole, and counter rotation of ~ 5 km/s were observed, indicating that the Local arm has similar kinematic properties as found for other major spiral arms.

And, from the conclusion:

We have studied the nature of the Local arm by measuring parallax distances and proper motions of nine 22-GHz H2O masers associated with star-forming regions. We include previously published parallaxes and proper motions for 21 other star forming regions (from either H2O, CH3 OH, SiO masers or YSO continuum emission). These 30 sources clearly lie between the Sagittarius and Perseus spiral arms and belong to the Local arm. This arm is at least ∼5 kpc in length and ∼1 kpc in width; it is not a spur and may be a branch of the Perseus arm, a bifurcation of the Carina arm, or an independent arm segment. The average peculiar motions of the sources in the Local arm are similar to those of HMSFRs in other major spiral arms of the Milky Way.