Visibilidad Nacional e Internacional

The third approach is, by far and away, the most effective method of identifying nearby WDs. Because WDs are generally blue and faint, they can easily be confused with main-sequence stars of similar temperatures that are much farther away. One way to minimize this confusion is to select objects that have sizeable proper motions (i.e.,µ >0.200yr−1_{). The assumption here is that if distant blue main-sequence objects}

have sizeable proper motions, their space motions would be large enough that they would no longer be bound to the Galaxy and should have left long ago. Presumably, if any remain today, these types of objects are rare and contribute negliglibly to surveys. Thus, we conclude that detected objects must be much less luminous, less distant, WDs. Of course, a more quantitative approach is necessary for this method to be useful.

Dating back to the early 20th century, before WDs were well understood, Ejnar Hertzsprung (1922) used a method now known as reduced proper motion (RPM) to separate dwarfs from giants. It basically serves to correlate proper motion with prox- imity. Though not entirely valid, it acts as a powerful diagnostic for assigning rough luminosity classes. At the time, its applicability to WDs was not yet conceived. The modern form of the equation is nearly identical to the absolute magnitude equation with µreplacing distance.

H =m+ 5 + 5 logµ, (3.1)

where m is the apparent magnitude of the object andµis its proper motion in units of arcsec yr−1_{. This equation effectively relates two observable quantities, apparent}

magnitude and proper motion, to a combination of two intrinsic properties, luminosity (absolute magnitude) and tangential velocity. We know that transverse velocity is related to distance by

VT = 4.74µd (3.2)

d= VT

4.74µ, (3.3)

where VT is an object’s transverse velocity in units of km sec−1 and d is its distance in pc. We can then incorporate this into the absolute magnitude equation,

m =M + 5 logd₋5, (3.4)

where M is absolute magnitude. Finally, we correlate RPM with tangential velocity as follows,

As a result, the RPM diagram is also efficient at separating high velocity objects such as halo/thick disk subdwarfs. Note, the term “subdwarf” as used here is very different than the hot sdO and sdB type subdwarfs mentioned in_§ 3.1 (see Jao et al. 2007, in preparation for a full discussion). Here, subdwarf refers to the old metal-poor stars likely not formed in the thin disk (luminosity class VI).

The most comprehensive proper motion catalog ever compiled for the northern hemisphere was done by L´epine & Shara (2005), called the LSPM-North catalog (to be discussed further in § 4.3). In it are 61,977 stars that have proper motions greater than 0.1500 _yr−1_{. Prior to this work, the proper motion catalog standard for}

25 years was the NLTT Catalogue (discussed in § 4.1). Also included is the best available magnitude and color information for each star, primarily from photographic plates. With these two pieces of information, the authors created a RPM diagram, shown in Figure 3.1. The WD region is clearly delineated from the halo/thick disk subdwarf region. There is significant contamination between the halo subdwarfs and the disk dwarfs. While no single object’s luminosity class is confirmed without follow- up spectroscopy, it is clear that this approach provides a vetted sample of promising WD candidates.

In an effort to identify new nearby WDs in the southern hemisphere, I used the RPM diagram as well. To date, no similar comprehensive proper motion catalog to the LSPM-North catalog has been compiled in the southern hemisphere. Therefore, I will briefly outline the various proper motion studies conducted in the south, as

Figure. 3.1: Reduced proper motion diagram for the 61,977 stars in the LSPM-North catalog. Clearly delineated are the WDs from the halo subdwarfs, which are much less delineated from the dwarfs. Reproduced from L´epine & Shara (2005).

well as give a detailed description of the proper motion survey we conducted, called the SuperCOSMOS-RECONS proper motion survey. I will then discuss my WD discoveries from the RPM diagram once these surveys have been addressed.

Chapter 4

Proper Motion Surveys

Stars have been known to exhibit proper motions for centuries. A small sample of stars have rather large proper motions. This quantity for a given star is usually the first measurement indicating that the star is nearby. This reasoning follows the analogy of a passenger riding in a car and looking out the window. The nearby light posts along the road pass very quickly, while the distant mountains in the background seem to move very slowly. Similarly, if a star has a very large proper motion, it seems to speed by while the background stars stay put. The difference in the analogy is that neither the light posts nor the mountains have any intrinsic motion; all of the perceived motion is because of the moving car. In contrast, not only is the Sun (our celestial car) moving, but also every other star in the Galaxy. Thus, proper motion is a convolution of our motion and the star’s intrinsic motion, which is why proper motion and distance are not perfectly correlated. Nonetheless, the compilations of proper motions have proved vital for detecting nearby stars.

Proper motion objects are primarily discovered by imaging the same piece of sky at least twice separated by several years in time. This technique of imaging for the sake of detecting proper motion stars first began near the turn of the 20th century (e.g., Kretz 1900). By blinking the two photographic plates, the high proper motion (HPM) objects were easily noticed because their positions change relative to the background stars. Because the motion is relative to the background stars, it is known

as relative proper motion. Another older technique used to discover HPM objects is with meridian circle observations (e.g. Tucker 1905). This technique uses a telescope that only observes the meridian and measures the precise time that a star crosses the meridian. The change in this measurement over time for a given object reflects the star’s proper motion. Because this measurement does not depend on background stars, it is considered an absolute measurement. The correction from relative to absolute depends on the average proper motion of the background stars and is on order of a few milliarcseconds (mas).